.  .    LIBRARY    .  . 

Connecticut 
Agricultural  College. 

VOL 2i...4 /.-.e^ — 

CLASS   NO .^....rr^.M 

COST .^.>*jt 

DATE (jilAJi....LS.». 19.0..^.. 


Digitized  by  the  Internet  Archive 

in  2009  with  funding  from 

Boston  Library  Consortium  IVIember  Libraries 


http://www.archive.org/details/mesabiironbearinOOIeit 


f.1 


vol 


4^ 


DEPARTMENT    OF   THE    INTERIOR 


MONOGEAPHS 


OF  THE 


United  States  Geological  Survey 


VOLUME    XLIII 


WASHINGTON 

GOVERNMENT     PRINTING     OFFICE 

19  0  3 


UNITED  STATES  GEOLOGICAL  SURVEY 

CHARLES  D.  WALCOTT,  PIEECTOE, 


THE 


MESABI  IRON-BEARING  DISTRICT  OF  MINNESOTA 


BY 


CHARLES  KENNETH  LEITH 


CHARLES    RICHARD    VAN    HISE,  Geologist  in  Charge 


WASHINGTON 

GOVERNMENT     PRINTINC4     OFFICE 

1  9  0  3 


9^  1  X.. 


CONTEI^TS. 


Page. 

Letter  of  transmittal 

Outline  op  monograph - 

Chapter  I. — Introduction - ^^ 

Geography  and  topography - ^'^ 

General  geology - 

Chapter  II. — Brief  history  of  district  and  summary  of  literature  concerning  it 25 

History  and  literature  of  the  district  prior  to  its  opening 25 

Opening  and  development  of  the  district - - 27 

Literature  on  the  district  subsequent  to  its  opening 29 

Summaries  of  literature,  arranged  chronologically - 31 

Chapter  III. — The  Basement  Complex,  or  Archean - 63 

Distribution -. -  -  -  - - - - ^^ 

Kinds  of  rocks - ^^ 

Dolerites  and  metadolerites -. ^4 

Basalts  and  metabasalts - 64 

Diorites - ^^ 

Peridotite - ^^ 

Hornblendic  schists 66 

Micaceous  schists  and  chloritic  schists 68 

Granite  and  porphyritic  rhyolite , 68 

Sedimentary  rocks - 69 

Structure ''^ 

Relations  to  other  series' '^ 

Chapter  IV. — The  Lower  Huronian  series - - 72 

Distribution - - '^ 

Kinds  of  rocks - - '"^ 

Gray wackes  and  slates '* 

Conglomerates - '  ^ 

Granites  and  porphyries - --- -  -- '^ 

Inclusions  in  granite °0 

Vein  cfuartz °'' 

Metamorphism  of  Lower  Huronian  rocks  by  granite - 83 

Relations  of  Lower  Huronian  granite  to  sediments  and  relations  of  both  to  other  series 84 

Structure ' - ' ^^ 

Thickness -  -  - ^'^ 

Chapter  V.— The  Upper  Huronian  series -  - 88 

Section  I.  Pokegama  quartzite _ - - - 90 

Distribution - - ^0 

Kinds  of  rocks ^^ 

Quartzite - 90 

Micaceous  quartz  slate - "^ 

Conglomerates - "* 

Structure 98 

Thickness - 99 

Relations  to  other  formations -  -  99 

5 


6         •  CONTENTS. 

Chapter  A'. — The  Upper  Hi-roxiax  series — Continued.-  Page. 

Section  II.  The  Biwabik  formation  (iron  bearing) 100 

Distribution 100 

Kinds  of  rocks lOl 

Greenalite  rocks 101 

Ferruginous,  amphibolitic,  sideritic,  and  calcareous  cherts 116 

Siliceous,  ferruginous,  and  amphibolitic  slates 143 

Paint  rock 149 

Sideritic  and  calcareous  rocks 150 

Conglomerates  and  quartzites 154 

The  alteration  of  the  iron  formation  by  the  intrusion  of  Keweenawan  granite  and  gabbro.  159 

Comparison  of  the  metamorijhic  effects  of  the  granite  and  gabbro 164 

Magnetic  attraction 164 

Structure 165 

Thickness - 166 

Relations  to  other  formations ■ 167 

Section  III.  Virginia  slate 168 

Distribution 168 

Kinds  of  rocks 169 

Slate 169 

Limestone 1~1 

Cordierite-hornstones  resulting  from  alteration  of  the  Virginia  slate  by  the  gabbro . .  171 

Relations  of  the  Virginia  slate  to  the  Biwabik  formation 172 

Comparison  of  slate  of  Virginia  and  Biwabik  formations 176 

Structure - 177 

Thickness 177 

Section  IV.  Structure  of  the  Upper  Huronian  series 1 78 

Section  V.  Thickness  of  the  Upper  Huronian  series ISO 

Section  VI.  Relations  of  the  Upper  Huronian  series  to  other  series 180 

Chapter  VI. — Keweenaw  an,  Cretaceous,  and  Pleistocene  rocks 182 

Section  I.  Keweenawan  rocks -- -  182 

Duluth  gabbro 182 

Contact  phases  of  gabbro 183 

Diabase - 185 

Embarrass  granite 186 

Proof  of  intrusion  of  Embarrass  granite  into  the  Upper  Huronian  series 187 

Section  II.  Cretaceous  rocks 189 

Fossils 190 

Section  III.  Pleistocene  or  glacial  deposits 191 

Chapter  VII. — Reslme  of  geologic  development  and  correlation 195 

Section  I.  Resum6  of  the  geologic  development 195 

Section  II.  Correlation 200 

Chaiteu  VIII. — The  iron-ore  deposits 206 

Distribution 206 

Shape 207 

Size 208 

Kinds  of  ore 209 

Minerals  and  rocks  contained  in  tlie  ore 210 

Chemistry 212 

Texture  and  structure 223 

The  rocks  forming  the  bottoms  and  sides  of  the  ore  deposits 227 

Structural  relations  of  tlie  ores  to  the  adjacent  rocks 227 

Petrographic  relations  of  the  ores  to  the  adjacent  rocks 233 

Drainage 234 


CONTENTS.  ^ 

Page. 

Chapter  IX. — Oeigin  of  the  iron  ores 23^ 

General  statement "''' 

Origin  of  the  greenalite  granules 239 

Greenalite  a  sedimentary  deposit 289 

Similarity  of  greenalite  to  glauoonite 239 

Composition  of  glauoonite  and  greenalite,  by  F.  W.  Clarke 243 

Explanation  of  the  occurrence  of  greenalite  in  granules - 247 

Manner  of  deposition  of  greenalite - 2o3 

I.  Develoi^ment  similar  to  glauconite r  -  -  - 25.3 

II.  Direct  precipitation  from  solution  by  organisms 2.55 

III.  Development  similar  to  that  of  iron  carbonate 255 

Iron  derived  from  the  weathering  of  preexisting  rocks  and  carried  to  the  ocean 

as  carbonate -'^^ 

The  iron  first  precipitated  in  the  ocean  as  hydrated  peroxide 256 

The  iron  first  precipitated  in  areas  of  vegetation 256 

The  hydrated  peroxide  reduced  by  vegetable  matter  and  the  protoxiile  of  iron 

combined  with  carbon  dioxide  or  silica 257 

Conclusion  with  reference  to  the  origin  of  the  greenalite  granules 259 

Burial  of  the  iron-bearing  formation  beneath  the  Virginia  slate 260 

Emergence  of  the  iron-bearing  formation  from  the  ocean 260 

Alteration  of  the  iron-bearing  formation  by  weathering  and  the  secondary  concentration  of 

the  ores '- 260 

Localization  of  ores  by  circulation  of  water 265 

Explanation  of  the  apparent  absence  of  ore  deposits  at  the  east  end  of  the  range 272 

Cause  of  distribution  of  phosphorus 274 

Points  of  similarity  and  difference  between  Mesabi  ores  and  those  of  other  Lake  Superior 

ranges 276 

Previous  explanations  of  the  origin  of  the  ore  and  their  relations  to  the  explanation  above 

given 27  ( 

Chapter  X. — Mining,  transpoetation,  production,  reserve,  ownership,  prices  of  ores,  fur- 
nace USE  op  ores 280 

Methods  of  mining 280 

Mining  by  steam  shovels  in  open  cuts 280 

MiUmg 282 

LTnderground  caving  and  slicing  systems 282 

Comparison  of  methods  of  mining - 283 

Transportation 285 

Production - 287 

Reserve  tonnage 290 

Ownership  and  control 291 

Price  of  Mesabi  ore  in  comparison  with  old  range  ores - 293 

Furnace  use  of  Mesabi  ores .., 294 

Chapter  XL — Exploration --' 295 

Index "^"3 


ILLUSTRATIONS 


Page. 

Plvte  I.  Sketch  map  of  Lake  Superior  region,   showing  iron  districts,  shipping  ports,  and 

transportation  lines -^^ 

II.  General  geologic  map  of  the  Mesabi  district In  pocket 

III.  First  train  load  of  Mesabi  ore,  1892,  on  Duluth,  Missabe  and  Northern  docks 28 

lY.  Detail  map  showing  distribution  of  Upper  Huronian,  Lower  Huronian,  and  Archean 

rocks  northwest  of  Hibbing ^° 

V.  Detail  map  showing  distribution  of  Upper  Huronian,  Lower  Huronian,  and  Archean 

rocks  north  of  jNIountain  Iron '" 

VI.  Detail   map   showing  distribution  of  Keweenawan,   Upper  Huronian,   and   Lower 

Huronian  rocks  in  the  vicinity  of  the  Mailman  camps 80 

VII.  Photomicrographs  of  normal  and  metamorphosed  Lower  Huronian  graywacke 82 

VIII.  Greenalite  rock ^^^ 

IX.  Photomicrographs  of  greenalite  granules 106 

X.  Ferruginous  chert  of  iron-bearing  formation 122 

XI.  Ferruginous  chert  of  iron-bearing  formation -  -  124 

XII.  Ferruginous  chert,  "jaspery"  phase,  and  ferruginous  chert  in  contact  with  quartzite 

of  iron-bearing  formadon 1"° 

XIII.  Photomicrographs  of  fresh  and  altered  greenalite  granules  and  ferrugmous  chert 

concretion 

XIV.  Photomicrographs  of  ferruginous  chert  granules,  showing  mottling 130 

XV.  Photomicrographs  of  ferruginous  chert  showing  ]ater  stages  of  the  alteration  of  green- 
alite granules -- 

XVI.  Photomicrographs  of  ferruginous  chert  of  Penokee-Gogebic  district -  -  134 

XVII.  Photomicrographs  of  ferruginous  and  amphibolitic  chert  of  iron-bearing  formation 

near  contact  with  Duluth  gabbro --_ l^^ 

XVIII.  Slate,  ferruginous  slate,  and  paint  rock  in  iron-bearing  formation  and  contact  of  iron- 
bearing  formation  with  intrusive  granite 152 

XIX.  Photomicrographs  showing  metamorphi-^m  of  Virginia  slate  into  cordierite-hornstone 

in  approaching  Duluth  gabbro 1 '  * 

XX.  Views  of  contact  of  the  ore  with  wall  rock  in  the  Biwabik  and  Mountain  Iron  mines.  232 

XXI.  Photomicrographs  of  granules  and  concretionary  structures  in  Clinton  iron  ores 250 

XXII.  Plan  of  tracks  in  the  Mahoning,  Mountain  Iron,  Fayal,  and  Biwabik  mines - .  -  280 

XXIII.  Map  of  Adams  mine  with  bottom  of  the  ore  deposit  indicated  by  contours 282 

XXIV.  A,  Railway  cut  in  approach  to  Oliver  mine  at  Virginia,  showing  close  jointing  and 

brittle  nature  of  the  iron-bearing  formation;   B,  Preliminary  stripping  at  Oliver 

mine,  Virginia "*'■* 

XXV.  Oliver  mine  at  Virginia  in  1900 - ----_-  284 

XXVI.  A,  Mountain  Iron  mine,  looking  north  through  mine;  B,  Steam  shovel  "bucking" 

bank  of  ore,  Mountain  Iron  mine ---  286 

XXVII.  A,  Stripping  operations,  Sharon  mine;  B,  View  of  Auburn  mine,  open  pit  and  shaft. .  286 

XXVIII.  A,  Panoramic  view  of  Adams  mine;  B,  Panoramic  view  of  Fayal  mine 292 

XXIX.  Adams  mine,  showing  horses  of  rock  in  open  pit 294 

XXX.  A ,  Mills  at  Fayal  mine;  B,  ililling  with  steam  shovel  at  Fayal  mine 294 


10  ILLCSTRATIOXS. 

Page. 
Plate  XXXI.  .1,  Saunti\\   mine,  showing  we-stward  monocliual  tilting  of  ore  strata:  B,  Faval 

mine,  showing  steam  shovel  "bucking"  l)ank  of  ore 294 

XXXII.   .1  and  B,  Panoramic  view  of  ilahoning  mine:   C  and  D,  Panoramic  view  of 

Bi wabik  mine 296 

XXXIII.  A,  View  of  Hale  mine,  showing  monocliual  dip  of  strata  of  ore  and  rock:  B, 

Duluth,  Missabe  and  Xortheru  docks  at  Duluth 298 

Fig.  1.  Detail  map  showing  distrilsution  of  I'pper  Huronian,  Lower  Hurrmian.  and 

-  Archean  rocks  northeast  of  E veleth 73 

2.  Sketch  of  contact  of  Lower  Huronian  granite  and  graywacke  siate.  showing  intri- 

cate nature  of  granite  intrusion 85 

3.  Detail  map  of  part  of  sec.  3,  T.  58  X.,  R.  17  W.,  showing  separation  of  quartzite 

at  the  base  of  the  Bi  wabik  formation  from  the  Pokegama  quartzite 89 

4.  Sketch  showing  distribution  of  the  Biwabik  and  Pokegama  formations  at  the  ■ 

lower  falls  of  Prairie  River 91 

5.  Sketch  showing  relations  of  Embarrass  granite  to  the  Biwabik  formation  in  the 

abandoned  glacial  gorge  in  the  XW.  -}  of  XAV.  \  of  sec.  17,  T.  60  AV.,  R.  12  W. .      160 

6.  Details  of  contact  of  Embarrass  granite  and  Biwabik  formation  in  the  gorge  in 

theXW.  iof  XW.  i-of  sec.  17,  T.  60  X.,  R.  12  W 161 

7.  Detail  map  showing  distribution  of  Lower  Huronian  granite,  Biwabik  formation, 

Keweenawan  gabbro  and  contact  phase  of  granite  with  gabbro  on  the  north 
shore  of  Birch  Lake 184 

8.  Sketch  showing  jiossible  connection  of  the  Mesabi  with  Penokee-Gogebic  iron- 

bearing  series 203 

9.  Ideal  cross  section  of  a  Mesabi  ore  deposit,  showing  relations  to  ferruginous  chert 

and  impervious  slate  layer 228 

10.  Ideal  section  parallel  to  the  pitch  of  a  Mesabi  ore  deposit,  showing  relations  to 

ferruginous  chert  and  to  impervious  slate  layer 229 

11.  Section  through  Biwabik  formation  transverse  to  the  range,  showing  nature  of 

circulation  of  water  and  its  relations  to  confining  strata 266 

12.  Sketch  showing  three  stages  in  the  downward  and  lateral  migration  of  an  ore 

deposit,  due  t-.j  the  truncation  of  the  iron  formation  by  erosion 270 


LETTER  OF  TRANSMITTAL. 


Department  of  the  Interior, 
United  States  Geological  Survey, 
Dr~-si0N  of  Pre-Cambrian  and  Metamorphic  GtEOLOGY, 

Madison,  Wis.,  June  23,  1902. 
Sir:  I  have  the  honor  to  transmit  herewith  the  manuscript  of  a  mon- 
ograph on  The  Mesabi  Iron-bearing  District  of  Minnesota,  by  Charles 
Kenneth  Leith.  Discovered  only  about  ten  years  ago,  in  the  early  nineties, 
the  Mesabi  district  has  to-day  no  rival  in  its  production  or  reserve  of  iron 
ore.  The  geological  succession  in  the  district,  the  unusual  size,  shape,  and 
structure  of  the  ore  bodies,  their  manner  of  development,  and  tlie  peculiar 
and  rajjid  methods  of  exploitation  of  the  ore,  all  present  features  of  unusual 
scientific  and  economic  interest. 

This  monograph  is  one  of  a  series  planned  to  treat  the  six  iron-bearing 
districts  of  the  Lake  Superior  region.  Monographs  on  the  Penokee- 
Gogebic,  Marquette,  and  Crystal  Falls  districts  have  been  published,  in  the 
order  named.  This,  on  the  Mesabi,  is  therefore  the  fourth.  The  last  two 
of  the  series  treat  of  the  Vermilion  and  Menominee  districts.  The  former, 
by  J.  Morgan  Clements,  is  ready  for  the  printer,  and  the  latter,  by  W.  S. 
Bayley,  is  nearing  completion. 

The  execution  of  the  full  plan  of  the  old  Lake  Superior  Division  of  the 
United  States  Geological  Survey  will  be  marked  by  the  ])ublication  of  a 
closing  monograph  entitled  The  Geology  of  the  Lake  Superior  Region, 
which  will  contain  a  general  discussion  of  the  geology  of  that  region, 
including  a  summary  treatment  of  each  of  the  iron-bearing  districts,  and  a 
general  discussion  of  the  ore  deposits.  This  monograph  is  in  preparation. 
Very  respectfully,  your  obedient  servant, 

C.  R.  Van  Hise, 
Geologist  in  Charge. 
Hon.  Charles  D.  Walcott, 

Director  of  United  States  Geological  Survey. 


11 


OUTLWE  OF  MONOGRAPH. 


Chapter  I  contains  a  general  account  of  the  geography,  topography,  and 
geology  of  the  Mesabi  district.  The  district  lies  northwest  of  Lake  Supeior  and 
extends  from  near  Grand  Rapids,  on  the  Mississippi  River,  a  little  north  of  east  to 
Birch  Lake,  a  distance  of  approximately  100  miles.  Its  width  varies  from  2  to  10 
miles,  and  the  total  area  is  about  400  square  miles.  The  main  topographic  feature  is 
a  ridge,  known  as  the  Giants  (or  Mesabi)  range,  which  extends  the  length  of  the 
district.  The  geologic  formations  represented  in  the  district  belong,  in  ascending- 
succession,  to  the  Archean,  Lower  Huronian,  Upper  Huronian,  Keweeuawan, 
Cretaceous,  and  Pleistocene.  Thej^  are  all  separated  by  unconformities.  The  core 
of  the  Giants  range  is  formed  by  Archean  and  Lower  Huronian  rocks,  except  for 
the  portion  in  ranges  12  and  13,  where  Keweenawan  gi-anite  forms  the  core.  On 
the  south  flank  rest  the  Upper  Huronian  rocks,  containing  the  iron-bearing 
formation,  with  gentle  southerly  dips.  The  Keweenawan  gabbro  lies  diagonallj- 
across  the  east  end  of  the  district.  The  Cretaceous  rocks  are  found  in  small  isolated 
patches  in  the  western  portion  of  the  district.  Pleistocene  drift  forms  a  more  or 
less  heavy  mantle  over"all  the  underlying  rocks. 

In  Chapter  II  is  given  the  historj^  of  early  explorations,  discover}^  of  ore,  and 
the  marvelous  economic  development  of  the  district,  together  with  summaries  of 
literature  on  the  geology  of  the  area. 

Chapter  III  treats  of  the  Archean  rocks.  They  consist  principally  of  green 
rocks  of  gi'eat  variety,  including  dolerites,  metadolerites,  basalts,  metabasalts, 
dioi'ites,  and  hornblendic,  micaceous,  and  chloritic  schists.  The  more  massive  rocks 
frequentlj^  have  an,  ellipsoidal  structure  which  is  characteristic  of  the  green  igneous 
rocks  of  other  parts  of  the  Lake  Superior  region.  In  addition  to  the  green  basic 
rocks  there  are  present  sniiall  areas  of  granite  and  porphyritic  rhyolite. 

In  Chapter  IV  the  Lower  Huronian  series  is  described.  The  series  consists  of 
sediments  and  granite.  The  sediments  are  graywackes,  slates,  and  conglomerates, 
all  metamorphosed,  with  bedding  and  schistosity  practicall}^  vertical.  Thej^  may 
be  as  thick  as  10,000  feet,  but  it  is  thought  more  probable  that  the  thickness  does 
not  exceed  5,000  feet.  The  Lower  Huronian  sediments  rest  unconformably  u^jon 
the  Archean  rocks,  as  shown  by  basal  conglomerates  containing  fragments  of  all 
the  varieties  of  rocks  found  in  the  Archean.  Previous  to.  the  work  done  in  con- 
nection with  the  preparation  of  this  monograph,  the  presence  of  the  Lower  Huronian 

13 


14  OUTLINE  OF  MONOGRAPH. 

series  of  sediments  had  not  been  determined,  but  everything  beneath  the  Upper 
Huronian  or  Animikie  had  been  mapped  as  Keewatin  or  Archean.  The  Lower 
Huronian  granite  forms  the  main  mass  of  the  Giants  range  westward  from  a  point 
near  the  east  line  of  R.  14  W.  It  is  intrusive  into  both  the  Archean  rocks  and  the 
Lower  Huronian  sediments  and  has  produced  strong  exomorphic  effects  in  both. 

Chapter  V  contains  an  account  of  the  Upper  Huronian  series,  of  importance 
because  it  includes  the  iron-bearing  formation.  The  series  consists  of  three  forma- 
tions— the  Pokegama  quartzite  at  the  base,  above  this  the  Biwabik  formation  (iron- 
bearing),  and  above  this  the  Virginia  slate. 

The  Pokegama  quartzite  (Section  I)  comprises  vitreous  quartzite,  micaceous 
quaitz-slate,  and  conglomerate.  The  thickness  ranges  from  0  to  500  feet,  averaging 
about  200  feet.  The  conglomerate  at  the  base  indicates  unconformable  relations  of 
the  Pokegama  formation  to  the  Archean  and  Lower  Huronian  rocks. 

The  Biwabik  formation  (Section  II),  the  iron-bearing  formation,  comprises 
ferruginous,  amphibolitic,  Sideritic,  and  calcareous  cherts,  siliceous,  ferruginous, 
and  amphibolitic  slates,  paint  rocks,  "  greenalite"  rocks,  sideritic  and  calcareous 
rocks,  conglomerates  and  quartzites,  and  iron  ores.  All  but  the  last  are  described  in 
this  chapter.  The  iron  ores  are  reserved  for  description  in  Chapter  VIII.  Cherts 
make  up  the  bulk  of  the  formation.  The  original  rock  of  the  formation  is  shown  to 
consist  largely  of  minute  granules  of  green  ferrous  silicate,  thus  conlirming  Spurr's 
conclusion.  The  material  was  called  glauconite  by  Spurr.  but  is  here  determined  to 
be  a  hydrous  ferrous  silicate  entirely  lacking  potash,  and  thus  not  glauconite.  It 
is  named  "greenalite"  for  convenience  in  discussion.  The  cherts  and  ii'on  ores  are 
shown  to  develop  mainly  from  the  alteration  of  the  greenalite  granules.  The  slates 
are  in  thin  laj'ers  interbedded  with  the  other  phases  of  the  iron  formation.  The 
paint  rocks  result  from  the  alteration  of  the  slates.  The  conglomerates  and  quartzites 
form  a  thin  layer  from  a  few  inches  to  perhaps  15  feet  or  more  in  thickness  at  the 
base  of  the  formation.  They  pass  upward  into  ferruginous  cherts  of  the  iron 
formation  rather  abruptly,  though  usually  at  the  contact  the  chert  and  quartzite  are 
interleaved  for  a  few  feet.  The  conglomerate  of  the  iron  formation  rests  upon 
Pokegama  quartzite,  indicating  a  slight  erosion  interval  between  the  Biwabik  and 
Pokegama  formations,  although  the  interval  is  not  shown  by  discoi'dance  in  bed- 
ding, which  is  parallel  in  both.  Heretofore  the  quartzite  and  conglomerate  in  the 
iron  formation  have  not  been  discriminated  from  the  rocks  of  the  Pokegama 
formation.  In  the  eastern  portion  of  the  range  the  iron  formation  is  in  contact 
with  the  Keweenawan  gabbro  and  granite,  and  near  this  contact  has  suffered  pro- 
found metamorphism.  The  characteristic  rocks  of  this  area  are  amphihole-mag- 
nctitecherts.  The  thickness  of  the  formation  may  vary  from  20(i  to  2,000  feet. 
The  average  may  bo  1,000  feet. 


OUTLINE  OF  MONOGRAPH.  15 

The  Virginia  slate  (Section  III)  is  essentially  a  soft  slate  or  shale  formation,  but 
it  contains  graywacke  phases,  near  its  base  a  little  limestone,  and  near  its  contact 
with  the  gabbro  is  metamorphosed  into  a  cordierite-hornfels.  The  noi'mal  slate 
phases  of  the  formation  may  be  distinguished  with  difficulty  in  isolated  occurrences 
from  the  slate  layers  in  the  Biwabik  formation.  The  separation  of  the  two  is  of 
importance  to  the  explorer,  and  hence  an  attempt  is  made  to  determine  criteria  for 
their  discrimination.  The  thickness  of  the  Virginia  formation  can  not  be  measured 
within  the  district,  but  from  analogy  with  the  Penokee-Gogebic  district  and  the 
extent  of  the  low,  flat-lying  area  south  of  the  Mesabi  range  supposed  to  be  occupied 
by  the  slate,  the  formation  is  believed  to  have  a  verj^  considerable  thickness.  The 
slate  grades,  both  vertically  and  laterally,  into  the  Biwabik  formation. 

The  structure  of  the  Upper  Huronian  series  is  described  in  Section  IV.  The 
entire  series  is  well  bedded,  conformable  in  structure  (although  having  a  thin 
conglomerate  between  the  Biwabik  and  Pokegama  formations),  and  dips  in  southerly 
directions  at  angles  varying  from  5  to  20  degrees  and  exceptionally  at  higher  or 
lower  angles.  The  series  is  gently  cross  folded  and  the  axes  of  the  cross  folds 
pitch  in  southerly  directions.  Accompanying  the  folding  is  considerable  jointing, 
especially  in  the  brittle  Pokegama  and  Biwabik  formations.  Indeed,  in  these  two 
formations  the  folding  is  l^rought  about  mainly  through  relatively  minute  displace- 
ments along  joints,  while  in  the  Virginia  formation  the  folding  has  taken  place 
mainly  bj'  the  actual  bending  of  the  strata. 

The  thickness  of  the  Upper  Huronian  series  (Section  V)  within  the  limits  of 
the  district  mapped  may  average  about  1,500  feet;  but  if  the  total  thickness  of  the 
slate  formation  outside  the  limits  of  the  district  be  taken  into  account,  the  total 
thickness  of  the  Upper  Huronian  series  is  probably  several  times  this  figure. 

The  relations  of  the  Upper  Huronian  series  (Section  VI)  to  the  subjacent  for- 
mations are  those  of  unconformity,  as  evidenced  by  basal  conglomerates,  discor- 
dance in  dip,  difl'erence  in  amovint  of  deformation  and  metamorphism,  distribution 
of  the  series,  and  I'elations  to  intrusives. 

In  Chapter  VI  the  Keweenawan,  Cretaceous,  and  Pleistocene  rocks  are 
described. 

The  Keweenawan  rocks  (Section  I)  consist  of  gabbro,  diabase,  and  granite,  all  of 
which  are  intrusive  into  the  rocks  with  which  they  come  into  contact.  The  north 
edge  of  the  gabbro  runs  diagonally  across  the  east  end  of  the  district  from  south- 
west to  northeast,  resting  upon  the  edges  of  each  of  the  members  of  the  Upper 
Huronian  series,  and  at  Birch  Lake  against  the  Lower  Huronian  granite.  North 
of  the  gabbro  margin  in  range  12  are  isolated  exjjosures  of  diabase  which  may 
represent  sills  associated  with  gabbro  intrusion.  The  granite  forms  the  crest 
of  the  Giants  range  through  ranges  12  and  13.     This  granite  has  not  heretofore 


1(3  OUTLINE  OF  MONOGRAPH. 

been   discriminated   from   tlie   Lower   Huronian   granite.     The  exomorphic   effect 
of  the  g-abbro  and  the  granite  upon  the  Upper  Huronian  series  has  been  profound. 

Cretaceous  rocks  (Section  II)  are  found  in  a  few  isolated  remnants  in  the  western 
portion  of  the  range.  They  consist  of  conglomerate  and  shale,  and  contain  fossils 
showing  them  to  belong  to  the  Upper  Cretaceous — not  older  than  the  Benton  and 
probablj'  not  younger  than  the  Pierre  horizon. 

The  Pleistocene  deposits  (Section  III)  form  a  heavj"  covering  over  the  district. 
On  the  upper  slopes  of  the  range  many  rock  exposures  project  through,  but  on  the 
lower  slopes  the  rock  series  are  commonly  buried  to  depths  ranging  from  20  to  150 
feet.  The  glacial  deposits  consist  of  stratified  and  unstratified  drift,  belonging 
principally  to  the  Itasca  and  Mesabi  moraines  of  the  latest  ice  incursion.  The 
movement  of  the  ice  was  mainly  from  northeast  to  southwest,  as  shown  by  glacial 
strife.  Several  remarkable,  steep-walled  gorges  through  the  crest  of  the  Giants 
range  at  high  elevations  are  believed  to  be  the  work  of  glacial  streams  escaping 
from  a  great  lake  ponded  between  the  Giants  range  and  the  ice  front  when  the 
glacier  had  drawn  back  north  of  this  area. 

Chapter  VII  contains  a  resume  of  the  geologic  development  and  a  correlation 
of  the  formations.  The  resume  of  geologic  development  is  itself  a  summary  and 
will  not  be  here  repeated.  The  essential  feature  of  the  correlation  is  the  equivalence 
of  the  Archean,  Lower  Huronian,  Upper  Huronian,  and  Keweenawan  series  with 
similar  sei'ies  in  other  parts  of  the  Lake  Superior  countrj-.  Prior  to  the  work  in 
the  Mesabi  district  done  in  connection  with  the  preparation  of  this  monograph, 
everything  below  the  Upper  Huronian  (or  Animikie)  had  been  mapped  as  Archean 
or  Keewatin.  The  work  of  the  Survej^  has  shown  the  supposed  Archean  or  Keewatin 
series  to  consist  of  two  series,  an  igneous  one  below  and  a  sedimentary  one  above, 
separated  by  a  profound  uncomformity,  to  be  correlated  respectively  with  the 
Archean  and  Lower  Huronian  series  of  other  parts  of  the  Lake  Superior  region. 
The  remarkable  similarity  of  the  Upper  Huronian  series  to  that  of  the  Penokee- 
Gogebic  district  is  again  emphasized  and  the  probability  of  their  area!  connection  is 
discussed. 

Chapter  VIII  is  a  description  of  the  iron-ore  deposits.  The  features  treated  are 
distri))ution,  shape,  size,  kinds,  minerals  and  rocks  contained  in  the  ore,  chemistry', 
texture  and  structure,  rocks  forming  the  bottoms  and  sides,  structural  relations  of 
the  ores  to  the  adjacent  rocks,  pctrographic  relations  of  the  ores  to  the  adjacent  rocks, 
and  drainage.  The  ores  are  in  basin-shaped  deposits,  with  great  variet}'  and  irregu- 
larity of  shape,  but  the  horizontal  dimensions  ai-e  usually  great  as  compared  with  the 
vertical  dimensions.  At  the  edges  the  layers  of  ore  grade  directly  into  the  layers 
of  wall  rock,  i)rincipally  ferruginous  chert.  The  deposits  lie,  for  the  most  part,  near 
tlie  axes  of  gentle  troughs  formed  by  the  folding  of  the  iron -formation  strata,  but  m 


OUTLINE  GF  MONOGRAPH.  17 

many  cases  the  strata  in  the  ore  and  adjacent  rocks  have  essential^  monoclinal  dips, 
indicating  the  deposit  to  be  independent  of  a  synclinal  structure  in  the  iron-forma- 
tion layers. 

Chapter  IX  contains  a  discussion  of  the  origin  of  the  ores.  The  ores  are  shown 
to  develop  mainly  from  the  alteration,  under  surface  conditions,  of  green  ferrous 
silicate  granules,  as  first  pointed  out  by  Spurr.  The  green  granules,  however, 
instead  of  being  glauconite,  as  maintained  bj^  Spurr,  are  believed,  from  their  lack  of 
potash,  to  be  of  different  nature,  and  have  been  given  the  name  greenalite.  Their 
development  is  believed  to  be  analogous  to  that  of  the  iron  carbonates  of  other  parts 
of  the  Lake  Superior  region.  That  is,  the  iron  was  carried  to  the  Upper  Huronian 
ocean  in  solution,  probably  as  carbonate,  was  precipitated  as  ferric  hydrate,  was 
buried  with  the  vegetable  material  and  reduced  to  the  protoxide  form,  and  was  then 
combined  with  silica  to  form  ferrous  silicate.  In  the  Gogebic  district,  where  silica 
was  not  present  in  so  great  abundance,  the  protoxide  combined  for  the  most  part 
with  carbon  dioxide  to  form  iron  carbonate.  The  shapes  of  the  granules  maj-  be 
due  to  replacement  of  minute  shells,  such  as  those  depositing  glauconite  or  those 
giving  shape  to  the  granules  of  much  of  the  Clinton  ore. 

The  secondar}'  concentration  of  ore  into  deposits  has  resulted  from  the  surface 
alteration  of  the  ferrous  silicate  (greenalite),  under  essentially  surface  conditions, 
since  the  iron  formation  was  first  exposed  to  weathering.  The  process  has  consisted 
essentially  in  the  decomposition  of  the  ferrous  silicate,  the  oxidation  of  the  protoxide 
of  iron  to  hematite  or  ferric  hydrate,  and  the  segregation  of  the  iron  and  silica. 
Where  this  has  occurred  on  a  small  scale,  banded  ferruginous  cherts  have  resulted; 
where  on  a  large  scale,  the  iron-ore  deposits  have  been  formed.  During  the  change 
both  iron  and  silica  have  been  carried  in  solution.  At  the  present  time  waters  flowing 
through  the  altered  portions  of  the  formation  are  concentrating  ore  by  the  solution 
and  abstraction  of  silica,  but  little  iron  being  carried  in  solution,  as  shown  b}'  analysis. 

The  localization  of  the  ores  by  circulation  of  underground  waters  is  described  in 
detail.  The  ores  are  shown  to  develop  both  above  and  below  ground  water,  to  have 
been  concentrated  on  impervious  basements  consisting  of  slaty  layers  in  the  iron 
formation,  which  have  limited  the  circulation  below,  and  to  have  been  in  part  confined 
to  troughs  formed  by  the  folding  of  such  impervious  strata  and  in  part  independent 
of  them.  They  have,  in  short,  developed  in  the  irregular  and  ramifying  channels  of 
water  circulation  in  gentlj'-  dipping,  much  jointed  strata.  Finally,  the  apparent  lack 
of  ore  deposits  in  the  iron  formation  far  under  the  edge  of  the  Virginia  slate  is 
shown  to  be  due  to  sluggish  circulation  under  the  slate  because  of  the  ponding  of 
water  under  that  impervious  formation. 

In  the  eastern  portion  of  the  range  the  iron  oxide  is  mainly  magnetite,  associated 
with  amphibolitic  chert,  and  has  not  yet  been  found  in  large  enough  deposits  to 
MON  XLIII — 03 2 


18  OUTLINE  OF  MONOGRAPH. 

warrant  mining.  The  explanation  of  the  nature  of  the  oxide  and  of  the  absence  of 
ore  deposits  in  this  portion  of  the  district  is  found  in  the  presence  of  the  Kewee- 
nawan  intrusives.  Prior  to  the  Keweenawan  intrusions  the  iron  formation  in  the 
eastern  end  of  the  district  had  been  exposed  to  erosion  by  the  removal  of  the  over- 
lying Virginia  slate.  In  the  central  and  western  portions  the  slate  had  not  been 
removed.  When  the  Keweenawan  rocks  were  intruded  the  iron-formation  rocks  of 
the  eastern  end  of  the  district  were  brought  under  deep-seated  conditions,  during 
which  the  changes  in  the  original  greenalite  rock  were  those  of  partial  oxidation  and 
silicitication,  resulting  in  the  production  of  amphibole-magnetite  rocks.  These  rocks 
are  stable  and  have  not  been  considerably  altered  since  their  exposure  to  surface 
alterations  by  the  erosion  of  the  Keweenawan  rocks.  The  rocks  of  the  iron  forma- 
tion in  the  central  and  western  portion  of  the  district  were  not  exposed  to  weathering 
agencies  until  after  Keweenawan  time,  when  the  Virginia  slate  had  been  removed  by 
erosion,  and  thus  were  never  metamorphosed  by  the  gabbro.  Their  alteration  has 
been  throughout  under  surface  conditions,  where  abundance  of  oxygen  and  carbon 
dioxide  makes  possible  the  complete  oxidation  of  the  iron  and  the  removal  of  silica 
on  a  large  enough  scale  to  cause  the  concentration  of  the  ore  bodies. 

During  the  development  of  the  iron-ore  deposits  erosion  has  continuously  cut 
down  the  iron  formation,  and,  because  of  the  gentle  dip  of  the  strata,  this  truncation 
has  been  accompanied  by  the  downward  and  lateral  migration  of  the  ore  deposits. 
The  deposits  in  their  present  position  may  be  supposed  to  represent  simply  a  stage 
in  the  process  of  concentration  and  migration.  Glacial  erosion  has  also  cut  down 
the  ore  deposits  to  a  considerable  extent. 

The  phosphorus  in  the  ores  is  shown  to  be  a  concentration  and  not  a  residual 
product.  The  original  greenalite  rocks  contain  little  or  no  phosphorus,  while  their 
altered  equivalents,  the  ores  and  cherts,  uniformly  show  small  contents  of  phos- 
phorus. The  unaltered  iron-formation  slates  also  contain  a  lower  percentage  of 
phosphorus  than  their  altered  equivalents,  the  paint  rocks. 

The  points  of  similarity  and  difference  between  the  Mesabi  ores  and  those  of 
other  Lake  Superior  ranges  are  briefly  summarized. 

Previous  explanations  of  the  origin  of  the  ore  are  outlined  and  their  relations  to 
the  present  explanation  shown. 

Chapter  X  is  devoted  to  economic  features  of  interest,  such  as  mining,  trans- 
portation, production,  reserve,  ownership,  prices,  etc.  The  open-pit  steam-shovel 
method  of  mining,  characteristic  of  the  district,  is  illustrated. 

In  Chapter  XI  exploration  for  ore  is  discussed  and  an  attempt  is  made  to  give 
the  criteria  for  locating  explorations  which  the  geologic  structure  and  manner  of 
development  of  the  ore  would  seem  to  warrant. 


THE  MESABI  IRON-BEARING  DISTRICT,  MINNESOTA. 


By  Chakles  Kenneth  Leitij. 


CHAPTER   I. 

INTRODUCTION. 

The  following  mouograph  tells  of  the  geologic  and  economic  features 
and  of  the  sudden  and  gigantic  development  of  the  Mesabi  iron-bearing 
district  of  Minnesota,  the  sixth,  last,  and  greatest  of  the  iton  ranges  to  be 
discovered  in  the  United  States  portion  of  the  Lake  Superior  region. 

The  writer's  work  upon  the  district  has  been  done  under  the  super- 
vision and  advice  of  Prof  C.  R.  Van  Hise,  geologist  in  charge.  To  him 
is  due  in  large  measure  any  credit  this  report  may  deserve.  Indeed,  the 
work  has  been  largely  the  application  in  this  district  of  jjrinciples  and 
methods  of  work  developed  by  him. 

Field  work  in  the  Mesabi  district  was  begun  in  the  spring  of  1900  and 
continued  during  the  summers  of  1900  and  1901.  In  the  year  1901  M.  H. 
Newman  served  as  field  assistant.     Near  the  close  of  the  field  season  of 

1900  Professor  Van  Hise  spent  some  time  in  the  district  Immediately 
thereafter  he  and  the  writer  prepared  a  brief  preliminary  report  on  the 
district,  accompanied  by  a  geologic  map  of  the  central  portion,  which  was 
published  in  the  Twenty-first  Annual  Report  of  the  United  States  Geo- 
logical Sui'vey,  as  a  part  of  a  general  paper  on  Lake  Superior  iron-ore 
deposits,  by  Professor  Van  Hise.     At  the   close    of    the    field    season  of 

1901  the  preparation  of  the  final  report  and  maps  presented  herewith  was 
taken  up. 

The  topography  of  the  Mesabi  district  shown  on  the  accompanying 
maps  was  sketched  during  the  summers  of  1899  and  1900  by  a  party  of  the 
United  States  Geological  Survey  consisting  of  E.  C.  Bebb,  D.  L.  Fairchild, 

19 


20  THE  MESABI  IRON-BEARING  DISTRICT. 

Louis  B.  Weed,  aud  assistants,  in  charge  of  Mr.  Bebb.  Much  information 
concerning  section,  town,  and  range  lines  and  the  subdivisions  of  sections 
has  subsequently  been  furnished  by  Mr.  D.  L.  Fairchild,  who  has  had 
charge  of  parties  engaged  in  locating  the  boundaries  of  properties  of  the 
Minnesota  Iron  Company.  This  information  has  applied  particularly  to 
the  area  between  Eveleth  and  Mesaba  station. 

Since  the  work  was  beg'un  in  the  Mesabi  district  the  Survey  has  had 
access  to  an  elaborate  set  of  maps  of  the  iron-beai'iug  series,  containing 
records  of  practically  all  of  the  exploration  work  done  on  the  range,  pre- 
pared by  J.  U.  Sebenius,  under  the  direction  of  W.  J.  Olcott,  for  the  Lake 
Superior  Consolidated  Iron  Mines,  now  a  part  of  the  United  States  Steel 
Corporation.  These  have  been  kept  up  to  date  by  Mr.  Sebenius  for  the 
United  States  Steel  Corporation.  Exploration  of  the  iron  formation  in 
the  Mesabi  district  is  done  largely  by  test  pitting  and  drilling  through  the 
glacial  drift  which  deeply  covers  the  district,  and  if  the  records  of  test 
pits  and  drill  holes  ai'e  not  collected  and  systematized  at  the  time  of  the 
exploration  the  information  is  largely  lost  for  purposes  of  mapping.  It  is 
apparent,  therefore,  that  accurate  mapping  of  the  iron  formation  would  have 
been  quite  impossible  without  access  to  such  maps  as  those  referred  to. 

Many  other  mining  men  and  explorers,  indeed  practically  all  interested 
in  the  ^lesabi  district,  have  given  the  Survey  information  concerning  their 
properties  and  have  placed  facilities  for  study  at  its  disposal.  The  wi'iter 
finds  himself  quite  unable  to  present  an  adequate  list  of  names  or  to  make 
a  satisfactory  selection  of  a  few  names  for  special  mention. 

The  mine  photographs  reproduced  were  furnished  by  W.  J.  Olcott,  E. 
E.  Sperry,  Greorge  Dormer,  E.  R.  Buckley,  and  local  professional  photog- 
raphers. 

To  all  whose  cooperation  has  aided  in  the  preparation  of  this  report 
the  Sui'vey  tenders  thanks. 

GEOGRAPHY  AND  TOPOGRAPHY. 

The  Mesabi  iron  district  lies  in  the  part  of  Minnesota  which  is  north- 
west of  Lake  Superior.  In  shape  and  trend  it  is  similar  to  the  other 
iron  districts  of  the  Lake  Superior  region  (see  PL  I).  It  extends  from 
Grand  Rapids,  on  the  Mississippi  River,  in  a  direction  ENE.  to  Birch 
Lake,  a  distance  of  approximately  KM)  miles,  with  a  width  varying  from  2 


GEOGRAPHY  AND  TOPOGRAPHY.  21 

to  10  miles.  Its  area  is  about  400  square  miles.  Eastward  from  Birch 
Lake  to  G-unflint  Lake  and  beyond  are  small  patches  of  iron-formation 
material,  and  these  areas  have  often  been  included  in  the  Mesabi  district, 
particularly  by  the  early  explorers. 

The  main  topogTaphic  feature  of  the  district  is  a  ridge  or  "range" 
parallel  to  the  longer  direction  of  the  district,  known  as  the  "Giants" 
or  "Mesabi"  range.  Mesabi"  (spelled  also  Mesaba  and  Missabe)  is  the 
Chippewa  Indian  name  for  "giant."  In  the  west  end  of  the  district  the 
Mesabi  range  merges  insensibly  into  the  level  of  the  surrounding  country, 
about  1,400  feet  above  sea  level,  or  800  feet  above  Lake  Superior.  Toward 
the  east  the  elevation,  with  reference  both  to  Lake  Superior  and  to  the 
surrounding  country,  increases;  from  range  18  to  range  12  elevations  of 
1,800  and  1,900  feet  above  sea  level,  or  400  and  500  feet  above  the  level 
of  the  surrounding  country,  are  reached.  For  many  miles  both  north  and 
south  of  the  range  there  is  a  comparatively  low,  flat  area,  and  the  Giants 
range,  particularly  its  eastern  portion,  is  a  very  conspicuous  feature  in  the 
landscape. 

While  the  general  trend  of  the  range  is  ENE.,  there  are  many  gentle 
bends  in  the  crest  line,  and  in  range  17  a  spur,  known  locally  as  the 
"Horn,"  projects  in  a  southwesterly  direction  for  6  miles.  The  crest  of 
the  range  is  in  places  broad  and  flat,  in  others  comparatively  narrow  and 
sharp.  The  southern  slope  is  very  gentle;  the  northern  slope  is  somewhat 
less  so.  At  frequent  intervals  both  crest  and  slopes  are  notched  by 
drainage  channels. 

The  Mesabi  range,  for  the  most  part,  forms  a  drainage  divide,  although 
it  is  crossed  by  drainage  channels  at  several  places.  The  drainage  of  the 
district  is  apportioned  among  three  of  the  great  river  systems  of  the 
country — the  Mississippi,  St.  Lawrence,  and  Nelson.  In  the  western 
portion  of  the  district,  from  Grand  Rapids  to  within  3  miles  of  Hibbing, 
the  southern  slope  is  drained  by  the  Mississippi  River  and  its  tributaries, 
the  Prairie  and  the  Swan.  The  Mississippi  and  the  Swan  cross  the  range. 
From  3  miles  west  of  Hibbing  to  east  of  Iron  Lake,  near  the  east  line  of 
range  13,  the  district  is  drained  to  the  south  by  the  St.  Louis  River  and 
its  tributaries,  the  Swan,  EmbaiTass,  and  Partridge      The  Embarrass  River 

"The  United  States  Board  on  Geographic  Names  has  adopted  this  spelling.  The  term  was  origi- 
nally applied  to  the  elevation  made  by  the  gabbro  eastward  from  Allen  Junction  to  Gunflint  Lake 
and  beyond,  but  has  since  been  applied  as  above. 


22  THE  MESABI  IRON-BEARING  DISTRICT. 

crosses  the  range.  The  St.  I^ouis  empties  into  Lake  Superior,  and  thence 
the  waters  of  the  system  are  contributed  to  the  St.  Lawrence.  From 
the  east  hue  of  range  13  to  east  of  Birch  Lake  the  district  is  di-ained  bv 
the  Dunka  River,  which  crosses  the  range  and  is  tributary  northward  to 
a  hike  system  which  discharges  through  Nelson  River  into  Hudson  Bav. 

The  northern  slope  of  the  Mesabi  range  is  drained  in  pait  bv  tlie 
Mississippi,  Prairie,  and  Embarrass  rivers,  flowing  south,  but  aside  from 
these  the  drainage  of  the  north  slope  all  goes  northward  into  the  lake 
system  tributarv  to  Hudson  Bay.  One  of  the  feeders  of  this  system,  the 
Pike  River,  reaches  well  down  into  the  southward-projecting  spur  of  the 
range  between  the  towns  of  Eveleth,  Vu'ginia,  and  McKinley,  thus  over- 
lapping the  headwaters  of  the  Embarrass. 

To  anyone  familiar  with  the  Lake  Superior  region  it  is  sufficient  to 
say  that  the  timber  and  soil  of  the  Mesabi  district  are  characteristic  of  the 
region.  The  forest  includes  the  white  pine,  the  yellow  or  Norway  pine, 
tamarack,  spruce,  cedar,  and  balsam  or  balsam  fir  (jack  pine).  Scattered 
among  them  are  hardwood  trees,  mainly  poplar,  birch,  and  maple.  For 
the  most  of  the  district  the  forest  is  essentially  coniferous,  Init  over  small 
areas  the  hardwood  trees  predominate.  Tamarack,  cedar,  and  spruce 
swamps  occupy  considerable  areas,  particularly  along  the  lower  slopes  of 
the  rang-e.  The  exceedingly  thick  underbrush  consists  largely  of  hazel, 
maple,  alder,  ash,  willow,  cherry,  and  ground  hemlock.  Most  of  the  pine 
has  been  cut. 

Old  choppings,  windfalls,  fires,  underbrush,  and  swamps  have  combined 
to  make  the  scene  a  desolate  one  for  much  of  the  district,  and  to  make  the 
ti-aveling  away  from  trails  or  roads  most  arduous.  In  the  limited  portions 
of  the  district  where  the  original  ])ines  still  stand,  all  the  beauties  of  the 
Northern  })ine  forest  at  its  best  are  to  be  observed. 

The  district  is  heavily  covered  with  glacial  drift,  consisting  of  sand, 
clay,  and  bowlders,  the  latter  in  some  places  so  ^•ery  numerous  as  to 
discourage  attempts  to  clear  the  land  for  agricultural  purposes.  L^p  to  the 
present  time  practically  no  land  has  been  cleared  outside  of  town  sites  and 
mining  locations.  There  are,  however,  considerable  areas  in  which  the  soil 
would  yield  abundantly  on  cultivation. 

Along  the  Mesabi  range  are  a  number  c>f  mining  towns,  most  of  tliem 
marking  mining  centers;    one  oulj',  Grand  Rapids,  on  the  west  end  of  the 


GENERAL  GEOLOGY.  23 

range,  owes  its  existence  to  lumber  interests,  and  even  this  town  is  benefited 
by  the  exploration  for  ore  in  the  western  portion  of  the  range.  The 
towns  are  largely  confined  to  the  central  portion  of  the  district.  Beginning 
at  the  Stevenson  mine,  in  R.  21  W.,  there  are  towns  at  frequent  intervals 
to  Mesaba  station,  in  range  14,  and  these  intervals  are  likely  to  be  further 
subdivided  as  the  exploitation  of  the  range  proceeds.  From  Mesaba  station 
to  the  east  end  of  the  range,  and  from  the  new  town  of  Nashwauk  to  Grrand 
Rapids,  near  the  west  end  of  the  range,  a  distance  in  each  case  of  a  little 
over  20  miles,  there  are  no  settlements. 

Three  railways,  all  with  terminals  on  Lake  Siiperior,  touch  the  range. 
The  Duluth  and  Iron  Range  Railway  crosses  the  district  in  R.  14  W.,  and 
sends  out  a  branch  to  Biwabik,  Stephenson,  McKinley,  Sparta,  Eveleth, 
and  Virginia.  The  Duluth,  Missabe  and  Northern  Railway  approaches  the 
range  from  the  south  through  R.  18  W.,  and  just  before  reaching  the  range 
sends  out  branches  to  Biwabik,  Eveleth,  Sparta,  Virginia,  Mountain  Iron, 
and  Hibbing.  The  Eastern  Railway  of  Minnesota  (the  Great  Northern) 
has  three  approaches  to  the  range,  one  through  R.  18  W.,  another  through 
Hibbing,  and  a  third  through  Grrand  Rapids.  Branches  connect  with 
Stevenson,  Chisholm,  Buhl,  Mountain  Iron,  and  Virginia.  The  only  large 
parts  of  the  range  not  immediately  accessible  by  railway  are  those  between 
the  Hawkins  mine  and  Grand  Rapids,  and  between  Mesaba  station  and 
Birch  Lake. 

GENERAL  GEOLOGY. 

The  succession  of  formations  in  the  Mesabi  district  appears  in  the 
following  table: 

Succession  of  formations  in  1  (esabi  district. 
Pleistocene. 

(Unconformity.) 
Cretaceous. 

(Unconformity.) 

Keweenawan Great  basal  gabbro  and  granite,  intrusive  in  all  lower  formations. 

(Unconformity.) 

Upper  Huronian  (Mesabi  series) Virginia  slate  (upper  slate  formation). 

(Unconformity.)  Biwabik  formation  (iron-bearing  formation). 

Pokegama  formation  (quartzite  and  quartz-slate  formation). 

Lower  Huronian Granite,  intrusive  in  lower  formations. 

(Unconformity.)  Slate-gray wacke-conglomerate    formation    (equivalent    to    the 

Ogishke  and  Knife  Lake  formations  of  the  Vermilion  district). 
Basement  complex,  or  Archean Greenstones,  including  basalts,  diorites,  diabases,  etc.,  hornblende- 
schists,  and  porphyritic  granites  and  rhyolites. 


24  THE  MESABI  IRON-BEARING  DISTRICT. 

The  core  of  the  Giants  range  is  made  up  principally  of  granite  of 
Lower  Hurouian  and  Keweenawan  age,  and  subordinately  of  Archean 
igneous  rocks.  To  the  south  of  the  igneous  core,  for  a  part  of  the 
district,  are  Lower  Huronian  sedimentary  rocks,  with  bedding-  approxi- 
mately vertical.  Against  the  southern  boundary  of  the  Lower  Huronian, 
or  where  the  Lower  Huronian  is  lacking,  against  the  igneous  core,  lie  the 
Upper  Huronian  sedimentary  rocks.  They  dip  gently  to  the  south  and 
underlie  the  greater  portion  of  the  southerly  slopes  of  the  range.  On  the 
southeast  the  Huronian  rocks  are  limited  by  the  Keweenawan  gabbro,  the 
north  edge  of  wliich  cuts  across  the  Huronian  formations  diagonally  from 
southwest  to  northeast.  The  Archean,  Lower  Huronian,  and  Upper 
Huronian  series  are  separated  from  one  another  by  unconformities.  Glacial 
drift  covers  the  district  so  thickly  that  rock  exposures  are  rare  on  the  lower 
slopes  of  the  range,  and  only  fairly  numerous  near  the  crest. 


CHAPTER  II. 

BRIEF  HISTORY  OF  THE  DISTRICT  AND  SUMMARY  OF 
LITERATURE  CONCERNING  IT. 

HISTORY  A3>f  D  LITERATURE  OF  THE  DISTRICT  PRIOR  TO  ITS  OPE]sri]srG. 

In  penetrating-  the  vast  wilderness  north  and  west  of  the  Great  Lakes 
country,  the  early  explorers  were  compelled  for  the  most  part  to  stick  close 
to  the  waterways,  for  the  nature  of  the  country  made  travel  for  long 
distances  exceedingly  arduous  by  any  other  method  than  canoeing.  Three 
of  the  canoe  routes  to  the  country  northwest  of  Lake  Superior  cross  the 
Mesabi  range  and  its  eastward  continuation  The  Mississippi  River  and 
its  tributaries,  the  Prairie  and  the  Swan,  touch  the  western  portion  of  the 
district.  Embarrass  Lake,  tributary  to  the  St.  Louis  River  and  thence  to 
Lake  Superior  and  the  St.  Lawrence,  crosses  the  Mesabi  range  near  its 
east-central  portion.  Gunflint  Lake,  one  of  a  chain  of  lakes  tributary  to 
Rainy  River  and  Nelson  River  and  thence  to  Hudson  Bay,  lies  far  to  the 
east,  on  a  continuation  of  what  is  now  known  as  the  Mesabi  range.  Thus 
it  is  that  the  first  references  to  the  Mesabi  district  found  in  literature 
concern  the  parts  of  the  district  immediately  adjacent  to  these  canoe  routes. 
Brief  descriptions  of  Pokegama  Falls  on  the  Mississippi  River  and  adjacent 
areas  were  made  by  Maj.  Z.  M.  Pike  in  1810,  by  Lieut.  James  Allen  and 
Henry  R.  Schoolcraft  in  1832,  and  by  J.  N.  Nicollet  in  1841.  In  1841 
also  Nicollet  published  his  map  of  the  hydrographic  basin  of  the  Upper 
Mississippi,  on  which  the  Mesabi  range,  called  "Missabay  Heights,"  was 
for  the  first  time  delineated,  by  hachures,  although  very  imperfectly.  In 
1852  J.  Gr.  Norwood  reported  the  occurrence  of  iron-formation  material  at 
Gunflint  Lake  and  mentioned  granite  and  gneiss  seen  in  crossing  the  range 
at  Embarrass  Lake.  In  1866  Col.  Charles  Whittlesey  reported  on 
explorations  made  in  northern  Minnesota  during  the  years  1848,  1859,  and 
1864.  He  mentioned  Pokegama  Falls  and  made  vague  reference  to  the 
granitic  rocks  of  the  range.     "Mesabi  range"  was  used  in  an  indefinite  way 


26  THE  MESABI  IRON-BEARING  DISTRICT. 

to  cover  what  are  now  known  as  the  Mesabi  and  VermiHon  ranges.  In 
1866,  also,  Hemy  H.  Eames,  the  first  State  geologist  of  Minnesota,  reported 
granite  and  gneiss  seen  on  a  trip  across  the  range  at  Embai-rass  Lake.  In 
describing  the  ranges  of  the  northern  part  of  the  State,  including  tlie 
"  Missabi  Wasju,"  he  stated  that  they  appear  to  be  traversed  by  metal-bearing 
veins.  Presumably,  liowever,  this  statement  refei's  mainly  to  the  Vermilion 
range.  In  a  second  report,  published  the  same  year,  Mr.  Eames  is  more 
explicit,  and,  referring  to  the  general  elevated  area  of  the  northern  part  of 
the  State,  including  the  Mesabi  range,  states:  "In  this  region  are  found 
also  immense  bodies  of  the  ores  of  iron,  both  magnetic  and  hematitic, 
occurring  in  dikes  and . associated  with  the  rock  in  which  it  is  found;  in 
some  of  these  formations  iron  enters  so  largely  into  its  composition  as  to 
affect  the  magnetic  needle."  Pokegama  Falls  and  Prairie  River  Falls  were 
visited,  and  at  the  latter  place  the  presence  of  "  iron  ore"  was  noted.  These 
reports  of  Eames  contain  the  first  references  to  iron  ore  in  the  Mesabi 
district  proper,  although  iron  formation  had  been  noted  by  Norwood  in 
1862  at  Gunflint  Lake 

From  this  time  on  desultory  exploration  work  was  done  in  certain 
portions  of  the  district.  It  was  confined  for  the  most  part  to  the  area 
west  of  Bii-ch  Lake,  in  Rs.  12,  13,  and  14  W.,  and  to  the  vicinity  of  the 
Prairie  River.  No  published  accounts  of  the  earlier  portion  of  this  explo- 
ratory work  are  to  be  found. 

The  fij'st  examination  of  the  Mesabi  range  by  a  mining  expert  with 
particular  reference  to  the  occurrence  of  iron  ore  in  workable  deposits,  noted 
in  print,  was  made  in  1875  by  Prof.  A.  H.  Chester,  of  Hamilton  College,  New 
York.  Striking  the  Mesabi  range  at  Embarrass  Lake,  he  worked  eastward 
toward  Birch  Lake.  In  his  report  (published  in  1884)  he  called  attention 
to  the  magnetic  character  of  the  iron  in  this  area,  and  to  the  fact  that  the 
alternating  iron  layers  are  not  thick  or  continuous.  The  percentage  44.G8 
was  given  as  a  fair  average  of  iron  in  the  rocks  of  this  part  of  the  district. 
In  general,  one  gathers  the  impression  that  he  was  not  favorably  impressed 
with  the  economic  prospects  of  this  area.  Between  the  time  of  Professor 
Chester's  examination  of  the  range,  in  1875,  and  the  i)ublication  of  his  report, 
in  1884,  Prof  N.  H.  Winchell,  State  geologist  of  Minnesota,  bi-iefly  noticed 
the  Mesabi  range  in  two  of  liis  reports.  In  1879  he  told  of  the  occurrence  of 
iron  ore  in  R.  14  W.,  and  pul)lished  analyses.     In  1881  he  told  of  a  trip  from 


OPENING  AND  DEVELOPMENT  OF  THE  DISTRICT.  27 

Embarrass  Lake  east  to  I'ange  14,  and  noted  the  magnetic  character  of  the 
iron  formation  in  range  14,  as  well  as  it^  similarity  to  the  formation  at 
Gmiflint  Lake.  Indeed,  the  iron  formation  in  range  14  is  called  the  "Gnn- 
flint  beds."  In  1883  Irving  called  the  Mesabi  iron-bearing  rocks  series 
"Animikie,"  a  term  which  had  been  applied  to  similar  rocks  at  Thunder 
Bay  and  westward  to  Gunflint  Lake,  and  correlated  the  Animikie  rocks 
with  the  "Original  Huronian"  rocks  of  the  noi-th  shore  of  Lake  Huron  and 
with  the  iron-bearing  series  of  the  Penokee-Gogebic  iron  range  of  Michigan 
and  Wisconsin.  From  this  time  on  the  term  "Animikie"  is  much  used  in 
the  literature  on  the  Mesabi  range  to  designate  the  iron-bearing  series. 
In  1 884,  in  the  same  report  in  which  Chester's  report  was  published, 
N.  H.  Winchell  discussed  the  age  of  the  Mesabi  series  of  rocks,  assigning 
them  to  the  "Taconic,"  or  Lower  Cambrian,  and,  following  Irving,  correlated 
them  with  the  iron-bearing  rocks  of  the  Penokee-Gogebic  district.  In  the 
late  eighties  a  numbei*  of  other  reports  on  the  district  were  issued  by  the 
Minnesota  survey,  but  they  contain  no  important  points  not  noted  in  reports 
above  cited.     This  brings  us  to  the  opening  of  the  district  for  mining. 

OPENIJS^G  AKD   DEVEIjOPMEIS^T   OF  THE   DISTRICT. 

Since  the  late  sixties  there  had  been  more  or  less  exploration,  partic- 
ularly along  the  eastern  portion  of  the  district,  from  Embai'rass  Lake  to 
Birch  Lake,  and  the  presence  of  iron-formation  material  had  been  recognized 
and  discussed  in  the  reports  above  mentioned.  However,  not  a  single 
deposit  of  ii'on  ore  of  such  size  and  character  as  to  warrant  mining  had 
been  shown  up.  In  fact,  the  range  had  been  "turned  down"  by  many 
mining  men  who  had  examined  it.  This  was  largely  because  of  the  fact 
that  they  confined  their  attention  principally  to  the  eastern,  magnetic  end 
of  the  range,  where  exposures  of  the  iron  formation  are  numerous.  Even 
up  to  the  present  time  no  ore  has  been  found  there  in  quantity.  Yet  the 
impression  was  gradually  developing  that  iron  ore  in  large  quantity  was  to 
be  found  in  this  district,  and  a  few  prospectors  were  working  diligently. 

Among  the  more  persistent  of  the  Mesabi  range  explorers  were  the 
Merritts — Lon  Merritt,  Alfred  Merritt,  L.  J.  Merritt,  C.  C.  Merritt,  T.  B. 
Merritt,  A.  R.  Merritt,  J.  E.  Merritt,  and  W.  J.  Merritt— of  Duluth,  Minn. 
Their  faith  in  the  range  was  the  first  to  be  rewarded.  On  November  16, 
1890,  one  of  their  test  pit  crews,  in  charge  of  Capt.  J.  A.  Nichols,  of  Duluth, 


28  THE  MESABI  IRON-BEARING  DISTRICT. 

struck  iron  ore  in  the  NW.  ^  sec.  3,  T.  58  N.,  R.  18  W.,  just  north  of  what 
is  now  known  as  the  Mountain  Iron  mine.  This  was  followed  in  1891  by 
the  discovery  of  ore  in  the  area  now  covered  by  the  Biwabik  and  Cincin- 
nati mines.  John  McCaskill,  an  explorer,  observed  iron  ore  clinging  to  the 
roots  of  an  upturned  tree  on  what  is  now  the  Biwabik  property.  This  led 
to  test  pitting,  and  test  pitting  by  the  Merritts  on  the  area  of  the  Biwabik 
mine,  under  charge  of  "W.  J.  Merritt,  led  to  the  discovery  of  this  mine  in 
August,  1891.  The  Cincinnati  mine  was  opened  the  same  fall.  The  Hale, 
Kanawha,  and  Canton  mines  were  shown  up  in  the  spring  of  1892. 

The  discovery  of  ore  near  what  are  now  known  as  the  towns  of 
Virginia,  Eveleth,  McKinley,  and  Hibbiug  followed  in  rapid  succession. 
The  excitement  following  the  first  discovery  of  ore  at  Mountain  Iron  was 
greatly  augmented  by  each  succeeding  find,  and  in  1891  and  1892  there 
was  the  inevitable  rush  of  explorers. 

Up  to  October,  1892,  there  were  two  railways  touching  the  range,  the 
Duluth  and  Iron  Range,  crossing  the  range  at  Mesaba  station  on  its  way  to 
the  Vermilion  range,  and  the  old  Duluth  and  Winnipeg  (now  the  Great 
Northern),  reaching  the  range  at  Gi'and  Rapids.  Both  of  these  places 
were  far  removed  from  the  exploring  centers.  Most  of  the  explorers  went 
through  Mesaba  station.  Reaching  this  place  by  rail,  they  were  compelled 
to  travel  12  to  50  miles  to  the  west  along  "tote  roads,"  which  were  all  but 
impassable.  The  time,  money,  and  energy  needed  to  conduct  even  modest 
explorations  at  this  time  can  be  appreciated  only  by  those  who  have 
experienced  the  difficulties  of  inland  travel  in  the  Lake  Superior  region 
away  from  railways.  The  stories  of  this  "toting"  period  contain  the 
usual  records  of  misfortunes,  lucky  strikes,  and  enterprise  incidental  to  a 
mining  boom. 

The  railways  were  not  long  in  getting  into  the  field.  In  October, 
1892,  two  lines  were  put  in  operation.  The  Duluth,  Missabe  and  Northern 
Railway  was  built  to  connect  Mountain  Iron  mine  with  the  old  Duluth 
and  Winnipeg  Railway  (now  the  Eastern  Railway  of  Minnesota,  a  part  of 
the  Great  Northern  system)  at  Stony  Brook  Junction,  and  later  was 
extended  to  Duluth.  Almost  immediately  after  the  connection  with  Mountain 
Iron  a  branch  was  sent  out  to  Biwabik.  About  the  same  time  the  Duluth 
and  Iron  Range  Railway  sent  out  a  branch  from  its  main  line  to  the  group 
of  mines  at  Biwabik.     Very  soon  thereafter  both  railways  got  into  Virginia. 


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DEVELOPMENT  OF  THE  DISTRICT.  29 

Hibbing  was  reached  by  the  Duluth,  Missabe  and  Northern  m  1893. 
Eveleth  was  reached  by  the  Duluth  and  Iron  Range  in  1894,  and  by  the 
Duluth,  Missabe  and  Northern  ver}:^  soon  thereafter.  The  Mississippi  and 
Northern  (Eastern  Railway  of  Minnesota)  about  the  same  time  projected 
a  spur  from  Swan  River  to  the  Hibbing  district. 

With  the  advent  of  railways  the  development  of  the  range  went  on 
by  leaps  and  bounds.  This  marvelous  development  has  continued  to  the 
present  time.  The  only  considerable  check  occurred  during  the  period  of 
general  financial  depression  which  the  country  underwent  in  1894,  1895, 
and  1896.  Almost  an  untouched  wilderness  in  1890,  the  district  is  to-day 
the  greatest  producer  of  iron  ore  in  the  world.  The  rapidity  of  the 
development  of  the  mining  industry'  of  the  district,  carrying  with  it  all  the 
prosperity  of  the  range,  can  not  be  better  told  than  by  the  following  table 
of  shipments  from  the  district: 

Shipments  from  the  Mesabi  district." 

Gross  tons. 

1892 : . . .  4,  245 

1893 613,620 

1894 1,793,052 

1895 2,781,587 

1896 2,  882, 079 

1897 4,275,809 

1898 4,613,766 

1899 : 6,626,384 

1900 7,809,535 

1901 9,004,890 

1902 13,329,953 

Total 53,734,920 

LITEEATUKE   ON   THE   DISTRICT   SUBSEQUENT  TO   ITS   OPENING. 

With  such  a  phenomenal  development,  it  is  but  natural  that  the  litera- 
ture on  the  range,  published  since  its  opening,  should  be  voluminous.  The 
most  important  reports  have  been  issued  by  the  Minnesota  State  survey. 

In  1891  Messrs.  N.  H.  and  H.  V.  Winchell  published  jointly  a  general 
discussion  of  the  iron  ores  of  Minnesota.  This  was  written  for  the  most 
part  prior  to,  the  actual  discovery  of  ore  in  quantity  in  the  Mesabi  district, 
but  it  contained  also  a  brief  notice  of  the  Merritts'  discovery  of  iron  ore 
near  Mountain  Iron  in  1890.     A  general  account  of  the  structural  relations 

oOn  pp.  287-289  may  be  found  a  full  table  of  shipments  from  individual  mines.  All  figures  are 
taken  from  the  Iron  Trade  Beview,  except  for  1902. 


30  THE  MESABI  IRON-BEARING  DISTRICT. 

of  the  ores  of  the  Mesabi  range  was  given,  and  a  comparison  with  Vermilion 
ores.  The  iron  formation  was  described  and  mapped  as  extending  from 
Pigeon  River  to  the  Mississippi  River  on  tlie  west.  Detailed  descriptions 
were  given  of  the  principal  explorations  made  up  to  this  time,  and  the  report 
was  accompanied  by  a  map  showing  location  of  iron  indications  in  the 
district.  A  pi-ediction  was  made  that  the  Mesabi  range  would  be  found  to 
contain  vast  quantities  of  iron  ore.  The  first  report  on  the  Mesabi  district 
which  was  written  after  the  discovery  of  ore — written  largely  because 
of  the  discovery  of  ore — was  published  in  1893  by  H.  V.  Wiuchell.  This 
contained  a  map  of  the  part  of  the  range  then  pi-oductive,  and  a  general 
account  of  the  history  and  geology  of  the  range.  The  mining  developments 
up  to  that  time  were  fully  described.  In  1892  Van  Hise  correlated  the 
Auimikie  series  with  the  Upper  Huronian  division  of  the  Algonkian.  In 
1894  J.  E.  Spurr  issued  a" bulletin  containing  a  map  of  the  range  and  a  full 
discussion  of  the  geology,  and  maintained  that  the  ores  were  developed  from 
green  granules  which  he  called  glauconite.  This  was  the  first  serious  attempt 
to  determine  the  origin  of  the  ores.  His  conclusion  that  the  ores  are  mainly 
derived  from  green  granules  is  confirmed  by  the  work  reported  in  the 
following  monograph,  but  his  determination  of  the  green  granules  as 
glauconite  is  proved  to  be  erroneous.  In  1899  was  issued  Volume  IV  of 
the  Final  Report  of  the  Minnesota  survey,  containing,  besides  a  general 
map  of  the  Mesabi  range  and  its  eastern  extension  to  Gunflint  Lake,  a 
number  of  detailed  maps,  each  of  them  accompanied  by  a  description  of  the 
geology,  by  N.  H.  Wiuchell  or  U.  S.  Grant.  This  volume  was  followed  in 
1900  by  a  volume  (V)  containing  a  general  discussion  of  the  stratigraphic 
relations  of  the  rocks  of  northern  Minnesota,  including  the  Mesabi  rocks, 
by  N.  H.  Winchell.  In  1901  there  appeared  Volume  VI  of  the  same  survey, 
containing  practically  all  the  maps  published  in  previous  volumes  of  the 
survey,  and  one  new  one,  a  general  geological  map  of  the  State,  accompanied 
by  synoptical  descriptions. 

The  United  States  Geological  Survey  began  field  work  in  preparation 
for  this  monograph  on  July  1,  1900.  While  rather  voluminous  reports  on 
the  district  above  referred  to  had  already  appeared,  these  were  difterentiu 
scope  and  execution  from  the  United  States  Geological  Survey  series  of 
reports  on  the  iron  ranges,  and,  moreover,  they  contained  interpretation.s  of 
tlie  geology  of  the  area  with  which  the  United  States  Geological  Survev 


SUMMARIES  OF  LITERATURE.  31 

was  not  in  accord.  It  was  tlierefore  decided  to  map  and  report  on  the 
district. 

In  1901,  in  a  paper  on  the  iron-ore  deposits  of  the  Lake  Superior 
reg'ion,  by  C.  R.  Van  Hise,  there  was  included  a  preliminary  report  on  the 
geology  of  the  Mesabi  district,  by  C.  R.  Van  Hise  and  C.  K.  Leith,  accom- 
panied by  a  map  of  part  of  the  district  by  C.  K.  Leith.  This  preliminary 
report  contains  many  of  the  essential  features  of  the  following  monograph. 

In  addition  to  the  above  reports  there  have  appeared  a  considerable 
number  of  articles  concerning  the  economic  features  of  the  district,  some  of 
which  are  listed  on  pages  61  and  62. 

STJMMARrES  OF  LITERATUKE,  ARRANGED  CHROKOLOGICAliLT. 

In  the  following  summaries  of  literature  on  the  Mesabi  range  no  refer- 
ence is  made  to  a  number  of  geological  reports  containing  general  discus- 
sions or  incidental  references  to  the  general  geology  of  northern  Minnesota. 
Neither  is  any  attempt  made  to  summarize  all  the  articles  on  the  economic 
features  of  the  Mesabi  district  which  have  appeared  in  the  publications  of 
engineering  a,nd  mining  societies  and  trade  journals  the  world  over.  The 
titles  of  some  of  these  articles  appear  on  pages  61  and  62.  Only  such 
reports  are  summarized  as  mark  the  historical  development  of  knowledge 
of  the  range. 

The  reader  is  likely  to  have  difficulty  in  understanding  some  of  the 
geological  names  used  in  reports  summarized.  A  variety  of  terms  have 
been  used  for  the  same  formations,  and  the  same  term  has  occasionally 
been  used  with  different  meaning  by  different  writers.  The  meaning  of 
the  names  is  discussed  in  the  section  on  Correlation  (pp.  200-205). 

isio. 

Pike,  Maj.  Z.  M.  An  account  of  expeditions  to  the  sources  of  the  Mississippi 
and  through  the  western  part  of  Louisiana,  performed  by  order  of  the  Government 
of  the  United  States  during  the  years  ISOo,  1806,  and  1807.     Philadelphia,  1810. 

A  brief  mention  of  Pokegaraa  Falls  is  here  made. 

1833. 

Allen,  Lieut.  James.  Report  of  Schoolcraft's  expedition  of  1832  to  the 
sources  of  the  Mississippi  River.  American  State  Papers,  Vol.  V,  Military  Affairs, 
page  330.     1833. 


32  THE  MESABI  IRON-BEARING  DISTRICT. 

Lieutenant  Allen  briefly  describes  Pokegama  Falls,  and  states  tliat  the 
river  breaks  through  a  low  ridge  which  trends  northeast-southwest.  This 
is  the  first  mention  of  the  ridge  which  was  later  known  as  the  Mesabi. 

1841. 

Nicollet,  J.  N.  Report  intended  to  illustrate  a  map  of  the  hydrographical 
basin  of  the  Upper  Mississippi  River,  made  while  in  the  emploj'  of  the  Bureau  of 
the  Corps  of  Topographical  Engineers.  Twenty-sixth  Congress,  2d  session,  1841, 
Senate  Document  No.  237,  series  No.  380,  p.  63. 

The  rocks  at  Pokegama  Falls  are  briefly  described  and  Missabay 
Heights  are  indicated  on  the  map  for  the  first  time  by  Hachures. 

issa.  V 

Norwood,  J.  G.  Geology  of  the  northwestern  and  western  portion  of  the 
valley  of  Lake  Superior.  Published  in  the  report  of  a  geological  suiwey  of 
Wisconsin,  Iowa,  and  Minnesota,  made  under  instructions  from  the  United  States 
Treasurj'  Department,  by  David  Dale  Owen,  Philadelphia,  1852,  pages  213-418. 

Dr.  Norwood  crossed  the  Mesabi  range  in  two  places — at  Grunflint 
Lake  and  at  Embarrass  Lake.  He  reports  the  occuiTonce  of  iron-formation 
material  on  Grunflint  Lake.  Groing  north  across  the  Saganaga  granite,  he 
makes  the  statement  that  this  granite  range,  if  continued  in  a  southwesterly 
direction,  would  pass  in  the  direction  of  the  Missapi  Wachu  and  Pokegama 
Falls  on  the  Mississippi  (p.  417). 

In  going  up  the  north  shore  of  Embarrass  Lake  he  crossed  the  Mesabi 
range  and  observed  syenitic  granite  associated  with  gneiss. 

1855. 

ScHOOLCKAFT,  Henry  R.  Summary  narrative  of  an  expedition  to  the  sources 
of  the  Mississippi  in  1820,  resumed  and  completed  upon  the  discovery  of  its  origin 
in  Itasca  Lake  in  1832,  with  appendices. 

This  contains  a  brief  mention  of  Pokegama  Falls. 

1866. 

Eames,  Henry  H.  The  metalliferous  region  bordering  on  Lake  Superior. 
First  Report  of  the  State  Geologist  of  Minnesota,  St.  Paul,  1866,  pages  8  and  9. 

In  1865  Eames  crossed  the  "Missabi  Wasju"  at  Embarrass  Lake,  and 
in  his  report  mentions  the  granite  there  seen. 

Eames,  Henry  H.  Geological  reconnoissance  of  the  northern,  middle,  and 
other  counties  of  Minnesota.  Second  Report  of  the  State  Geologist  of  Minnesota, 
St.  Paul,  1866.  pages  1-58. 


SUMMARIES  OF  LITERATURE.  33 

Eames  in  1866,  referring  to  the  "gigantic  uplifts"  in  the  northern 
part  of  the  State,  which  reach  their  greatest  altitude  "  at  or  near  Missabe 
Heights,"  states:  "In  this  region  are  found  also  immense  bodies  of  the 
ores  of  iron,  both  magnetic  and  hematitic,  occurring  in  dikes  and  asso- 
ciated with  the  rock  in  which  it  is  foiind.  In  some  of  these  formations 
iron  enters  so  largely  into  its  composition  as  to  affect  the  magnetic  needle, 
both  in  its  horizontal  deflection  and  vertical  dip"  (p.  18). 

In  a  geological  reconnaissance  Pokegama  Falls  and  the  falls  of  Prairie 
River  were  visited.  The  rock  at  Pokegaina  Falls  is  referred  to  the  Pots- 
dam. The  lower  or  first  falls  of  Prairie  River  were  referred  to  as  "  an 
uplift  of  igneous  and  metamorphosed  rocks,  consisting  of  granite,  coarse 
and  fine,  quartzite  or  Potsdam  sandstone,  and  iron  ore,"  which  occurred  in 
the  following  order  in  ascending  the  river : 

1.  Fine-grained  quartzose  granite. 

2.  Iron  ore. 

3.  Quartzite  (Potsdam  sandstone). 
4:  Fine  granite. 

6.  Primitive  schistose  rock. 

6.  Argillaceous  slate. 

Passing  the  first  falls  near  the  upper  end  of  the  lake  on  the  south  side, 
an  uplift  is  seen  50  feet  high,  showing  the  same  succession  of  strata  as 
above  given,  beginning  with  No.  4  and  ending  with  No.  1.  Nos.  5  and  6 
are  not  seen  here. 

The  upper  falls  of  the  Prairie  River  cut  through  an  uplift  of  granite 
rocks. 

Whittlesey,  Col.  Charles.  A  report  of  explorations  in  the  mineral  region 
of  Minnesota  during  the  years  ISiS,  1859,  and  1864.     Cleveland,  1866. 

Colonel  Whittlesey  gives  a  vague  account  of  the  northern  portion  of 
Minnesota,  using  the  term  "Mesabi"  range  to  cover  the  general  elevated 
area  in  the  northern  part  of  the  State,  including  the  Vermilion  range. 
The  region  is  referred  to  as  "  an  imperfectly  defined  region  of  granite, 
syenite,  mica  slate,  siliceous,  and  talcose  rocks  extending  to  and  across  the 
national  boundary." 

The  quartzite  of  Pokegama  Falls  is  referred  to  the  Potsdam  sandstone. 
MON  XLIII — 0.3 -3 


34  THE  MESABI  IRON-BEARING  DISTRICT. 


1879. 


AViNCHELL,  N.  H.     Sketch  of  the  work  of  the  season  of  1878.     Geological  and 
Natural  Histor}'  Survey  of  Minnesota,  Seventh  Annual  Report,  1879,  pages  9-25. 

Professor  Wiuchell  here  tells  of  the  occuiTence  of  iron  ore  near  Grun- 
flint  Lake  and  in  Ts.  59  and  60  N.,  R.  14  W.,  and  publishes  two  analyses  of 
ore  from  range  14  (pp.  22,  23).  The  "  Mesabi  Heights"  are  referred  to 
as  due  to  drift  and  to  hard  quartzite. 


1881. 


WiNCHELL,  N.  H.     Geological  and  Natural  History  Survey  of  Minnesota,  Ninth 
Annual  Report,  1881,  pages  106-109. 

Professor  "Winchell  writes  of  a  trip  down  the  Pike  River,  across  the 
portage  to  Embarrass  River,  down  the  Embarrass  to  the  dam,  thence  east 
to  range  14,  where  pits  in  iron-formation  material  (called  Gunflint  beds) 
are  reported  in  sections  14,  15,  and  28,  and  other  places.  The  magnetic 
character  of  the  formation  is  noted. 


1883. 


Irving,  R.  D.     The  copper-bearing  rocks  of  Lake  Superior.     Mon.  U.  S.  Geol. 
Survey  Vol.  V,  1883,  pages  382-384. 

In  his  famous  monograph  on  the  copper-bearing  rocks  of  Lake  Superior, 
Irving  correlates  the  Mesabi  iron-bearing  rocks  as  far  west  as  Pokegama 
Falls  with  the  slates  and  associated  iron-bearing  rocks  at  Gunflint  Lake  and 
thence  eastward  to  Thunder  Bay,  and  accordingly  calls  them  Animikie. 
In  1873  Hunt  had  suggested  the  name  Animikie  (Indian  for  Thunder  Bay) 
for  the  series  at  Thunder  Bay,  and  the  continuity  of  this  series  westward 
to  Gunflint  Lake  had  been  established  by  Bell"  and  Winchell." 

At  the  same  time  Irving  correlates  the  Animikie  with  the  original 
Huronian  of  the  north  shore  of  Lake  Huron  and  with  the  iron-bearing 
series  of  the  Penokee-Gogebic  district:  This  correlation  has  since  been 
accepted  by  Van  Hise  and  other  United  States  geologists,  and  it  is  the  one 
which  is  used  in  the  following  report. 

From  this  time  on  the  Mesabi  iron-bearing  series  (the  Upper  Huronian 
of  the  following  repoi-t)  is  frequently  referred  to  in  literature  as  the  Animikie 


«Geol  Surv.  of  Canada  for  1872-7.S,  by  Robert  Bell,  pp.  92-94. 

6  Ninth  Ann.  Kept.  Geol.  Nut.  Hist.  Survey  Minnesota,  by  N.  H.  Winchell,  1881,  p.  76. 


SUMMARIES  OF  LITERATURE.  35 

series,  altlioug'li  this  term  is  given  different  age  significance  by  different 
writers. 

The  petrography  of  the  gabbro  of  northeastern  Minnesota  is  fully 
described. 

1884. 

AViNCHELL,  N.  H.  The  geology  of  Minnesota.  Geological  and  Natural  His- 
tory Sui'vev  of  Minnesota,  Final  Report,  Vol.  I,  I88i. 

The  first  volume  of  the  Final  Report  of  the  Minnesota  survey  contains 
a  very  interesting  account  of  the  early  explorations  in  Minnesota.  The 
development  of  knowledge  of  the  geography  and  geology  of  the  State  is 
sketched.     Slight  reference  is  made  to  the  Mesabi  district. 

Following  the  historical  sketch  is  a  description  of  the  general  physical 
features  of  the  State.  The  discussion  of  the  drainage  systems  and  divides 
involves  the  discussion  of  the  topogi'aphy  of  the  Mesabi  district.  Timber, 
soils,  glacial  diift,  and  lakes  are  also  discussed. 

Chester,  A.  H.  The  iron  region  of  northern  Minnesota.  Geological  and 
Natural  History  Survey  of  Minnesota,  Eleventh  Annual  Report,  ISS-i,  pages  loi-167. 

Professor  Chester,  of  Hamilton  College,  in  1894,  published  an  account 
of  an  examination  of  the  eastern  end  of  the  Mesabi  range,  made  in  1875  for 
private  parties.  This  was  the  first  examination  of  the  range  by  a  mining 
expert  noted  in  literature.  Coming  from  Duluth  on  the  south,  he  struck  the 
Mesabi  range  at  the  portage  of  the  Embarrass,  and  worked  northeast 
along  the  range  to  T.  60  N.,  R.  12  W.  A  large  number  of  outcrops  and 
pits  were  examined.  The  siliceous  bands  associated  with  the  iron  bands  in 
the  iron  formation  are  called  quartzite.  Attention  is  called  to  the  magnetic 
character  of  the  iron  ore  and  to  the  fact  that  the  alternating  iron  layers 
are  not  thick  or  continuous.  From  the  samples  of  iron  ore  brought  back 
from  his  trip  a  large  number  of  analyses  were  made;  44.68  is  given  as  a 
fair  average  of  the  percentage  of  iron  in  the  part  of  the  district  covered. 
This,  of  course,  applies  to  iron  above  the  level  of  ground  water,  as  this  is 
the  depth  to  which  most  of  the  pits  were  sunk.  Professor  Chester  expressed 
the  opinion,  that  the  iron-bearing  rocks  of  the  Mesabi  district  bear  the 
same  relation  to  the  Huronian  rocks  as  do  the  rocks  of  the  Penokee  iron 
range  in  Wisconsin. 

To  the  north  of  the  magnetic  schists  constituting  the  iron  formation  is 


36  THE  MESABI  IRON-BEARING  DISTRICT. 

a  red  or  pink  granite  forming  the  backbone  of  the  Mesabi  range.  To  the 
north  of  this  ridge  the  rock  strata  are  much  more  inclined,  and  consist  of 
similar  slates  and  quartzite,  but  without  magnetite.  The  general  trend  is 
east  and  west,  following  the  trend  of  the  ridge.  The  second  belt,  the  red 
granite,  identical  in  appearance  with  that  of  the  Mesabi  backbone,  is 
exposed  on  the  long  portage  between  Embarrass  and  Pike  rivers  in  the 
southwestern  jiart  of  T.  60  N.,  R.  15  W. 

In  general  the  reader  gathers  the  impression  from  Professor  Chester's 
report  that  he  is  not  favorablj'  impressed  with  the  value  of  the  iron  ores 
from  the  part  of  the  district  ^"isited  by  him. 

WiNCHELL,  N.  H.     Geological  and  Natural  History  Survey  of  Minnesota,  Elev- 
enth Annual  Report,  1884,  pages  168-170. 

Professor  Winchell  discusses  the  age  of  the  Mesabi  rocks.  He  classes 
them,  with  the  Vermilion  iron-bearing  rocks,  as  Taconic,  or  Lower  Cam- 
brian, equivalent  in  part  to  the  Huronian  of  Michigan  and  Wisconsin. 

1885. 

Winchell,  N.  H.     Geological  and  Natural  History  Survey  of  Minnesota,  Thir- 
teenth Annual  Rei^ort,  1885,  pages  10-24. 

Professor  Winchell  here  describes  the  rocks  of  the  Mesabi  range  seen 
in  a  trip  across  the  range  along  the  Dnluth  and  Iron  Range  Railway. 
Gabbro  is  crossed  at  Okwanim  (Allen  Junction).  Two  miles  north  of 
here  is  a  cut  in  soft  reddish  iron-formation  material.  Four  miles  north  of 
Okwanim  is  gray  granite  or  syenite,  forming  the  Giants  rang-e.  This  is 
about  a  mile  wide.  Two  miles  to  the  north  is  red  granite  (Emban-ass). 
A  cross  section  shows  Huronian,  Animikie,  and  Gabbro  rocks  lying  with 
structural  conformity  with  one  another  on  the  syenite  of  the  Giants  range, 
to  the  north  of  which  is  shown  Huronian  conglomerate  and  greenstone, 
with  a  dip  to  the  north  of  about  the  same  degree  as  the  dip  of  the  strata 
south  of  the  range.  In  a  general  account  of  the  crystalline  rocks  in  the 
same  report  Professor  Winchell  describes  the  rock  succession  in  the  Mesabi 
and  Vermilion  ranges  as  follows: 

(1)  At  the  top  are  slates  and  quartzites,  with  beds  of  diorite  (the  Ani- 
mikie group).     These  contain  the  jMesabi  iron  ores. 

(2)  Soft  greenish  slaty  schists,  with  lenticular  masses  of  gneiss  and 
diorite.     These  contain  Vermilion  iron  ores  near  the  bottom. 


SUMMARIES  OF  LITERATURE.  37 

(3)  Conglomeratic  and  quartzitic  slates,  qnartzites,  and  marble;  per- 
haps contain  the  Vermilion  iron  ores. 

The  Mesabi  ores  are  probably  the  equivalent  of  the  Commonwealth 
ores  of  Wisconsin,  with  no  known  equivalent  in  Michigan. 

18S6. 

Willis,  Bailey.  Report  of  a  trip  on  the  Upper  Mississippi  and  to  Vermilion 
Lake,  Minnesota.     Tenth  Census  of  the  United  States,  Vol.  XV,  1886,  pages  457-i61. 

Mr.  Willis  briefly  describes  the  rocks  at  Pokegama  Falls  and  at  the 
two  falls  of  the  Prairie  River,  and  accompanies  the  description  with  a  sketch 
map  of  both  places. 

Irving,  Roland  D.  Origin  of  the  ferruginous  schists  and  iron  ores  of  the 
Lake  Superior  region.     Am.  Jour.  Sci.,  3d  series,  Vol.  XXXII,  1886,  pages  255-272. 

Irving  concludes  that  the  original  form  of  the  iron-bearing  rocks  of 
the  Lake  Superior  region  was  iron  carbonate,  and  that  the  iron  ores  and 
associated  rocks  of  the  iron  formation  have  resulted  from  the  alteration  of 
this  rock  by  percolating  waters.  With  reference  to  the  Gunflint  district  of 
the  Animikie  ores,  the  statement  is  made  that  a  study  of  slides  shows  "com- 
plete gradations  from  the  unaltered  carbonates  to  cherty  and  jaspery  mate- 
rials and  even  to  actinolitic  and  magnetite  schists "  (p.  262).  No  direct 
reference  is  made  to  the  ores  of  the  Mesabi  district. 

18S7. 

WiNCHELL,  N.  H.  Geological  and  Natural  History  Survey  of  Minnesota, 
Fifteenth  Annual  Report,  1887,  pages  209-399. 

In  this  report  Winchell  mentions  the  occurrence  of  nontitaniferous 
magnetite  in  T.  63,  R.  11  W.,  and  Ts.  59  and  60  N.,  R.  14  W.,  and  states 
that  it  is  comparable  to  the  "iron  ore  found  at  Black  River  Falls,  in  Wiscon- 
sin, and  at  the  western  end  of  the  Penokee-Grogebic  iron  range  on  the  south 
side  of  Lake  Superior"  (p.  216). 

1S8S. 

Winchell,  H.  V.  Geological  and  Natural  History  Survey  of  Minnesota, 
Sixteenth  Annual  Report,  1888,  pages  438^-10. 

Here  is  a  brief  description  of  Pokegama  and  Prairie  falls.  It  is  said 
that  the  Pokegama  were  formerly  much  higher  and  have  been  worn  down 
in  the  last  twenty  years.  The  Indians  call  them  "Kakabikag"  (rocky  falls). 
They  sometimes  add  a  diminutive,  and  call  them  the  "Little  Rocky  Falls." 


38  THE  MESABl  IKON-BEARING  DISTRICT. 

1S89. 

WiNCHELL,  H.  V.  Report  of  field  observations  made  during^  the  season  of  1888 
in  the  iron  reg'ion  of  Minnesota.  Geological  and  Natural  History  Survey  of  Minne- 
sota, Seventeenth  Annual  Report,  18S9,  pages  77-145. 

H.  V.  Winchell  here  reports  on  a  trip  from  Birch  Lake  southwest 
along  the  Dunka  River,  alouo-  the  Mesabi  range  to  the  Duluth  and  Iron 
Range  Railway,  and  back  again  to  Birch  Lake.  Descriptions  of  uumei'ous 
outcrops  of  the  iron  formation,  Giants  range  syenite,  and  the  gabbro  are 
given.  The  magnetic  character  of  the  iron  is  emphasized.  The  contact 
of  iron  formation  and  syenite  north  of  Iron  Lake  in  the  SE.  J  of  NE.  ^ 
sec.  35,  T.  61  N.,  R.  12  W.,  is  described.  The  iron  formation  rests  with 
normal  erosion  unconformity  upon  the  syenite. 

At  the  end  of  the  season  a  visit  was  made  to  Pokegama  Falls  and 
Prairie  River  Falls,  at  the  west  end  of  the  Mesabi  range,  but  no  new  points 
were  noted. 

Winchell,  N.  H.  Geological  and  Natural  History"  Survey  of  Minnesota, 
Seventeenth  Annual  Report,  1889,  pages  5-74.  See  also  the  Aniinikie  black  slates 
and  quartzites  and  the  Ogishki  conglomerate  of  Minnesota,  the  equivalent  of  the 
"Original  Hurouian."  Am.  Geol.,  Vol.  I,  pages  ll-ll.  Methods  of  stratigraphy 
in  studying  the  Huronian.     Ibid..  Vol.  IV..  pages  343-357. 

The  nontitaniferous  iron  ores  of  the  Mesabi  range  are  classed  with  the 
Taconic  and  placed  as  the  equivalents  of  the  Hui'onian  of  the  Marquette 
district,  the  Penokee-Gogebic  district,  the  Black  River  Falls  schists,  and 
the  Black  Hills  quartzites. 

X890. 

Winchell,  N.  H.,  and  Winchell,  H.  V.  The  Taconic  iron  ores  of  Minnesota 
and  of  western  New  England.     Am.  Geol.,  Vol.  VI,  1890.  pages  263-274. 

The  iron  ores  of  Minnesota  are  at  five  different  geolog'ical  horizons,  in 
descending  order,  as  follows:  (1)  The  hematites  and  limonites  of  the  Mesabi 
range,  the  equivalents  of  the  hematites  of  the  Penokee-Gogebic  range  in 
Wisconsin;  (2)  the  gabbro  titaniferous  magnetites  near  the  bottom  of  the 
rocks  of  the  Mesabi  range;  (3)  01i^'initic  magnetites,  just  below  the  gabbro 
in  the  basal  portion  of  the  Mesabi  rocks;  (4)  the  hematites  and  magnetites 
of  the  Vermilion  range  in  the  Keewatin  formation;  (5)  the  magnetites  of 
the  crystalline  schists  of  the  Vei'milion  formation.     It  is  maintained  that 


SUMMARIES  OF  LITERATURE.  39 

the  upper  iron  deposits  of  the  Mesabi  and  those  of  the  Penokee-Gogebic 
are  the  equivalents  of  the  Taconic  ores  of  western  New  England. 

In  the  fall  of  this  year  occurred  the  first  discovery  of  merchantable 
ore  in  the  Mesabi  district.     The  first  shipment  was  made  in  1892. 

1891. 

WiNCHELL,  N.  H.  Record  of  field  observations  in  1S88.  Geological  and 
Natural  History  Surve}^  of  Minnesota,  Eighteenth  Annual  Report,  1891,  pages  7-59. 

This  report  was  written  before  the  discovery  of  ore  in  the  district. 
Professor  Winchell  briefly  describes  the  explorations  of  John  Mailman 
2  miles  south  of  Hinsdale,  near  the  Duluth  and  Iron  Range  track,  calls 
attention  to  the  similarity  of  the  iron  foraiation  here  with  that  at  Grunflint 
Lake,  and  suggests  that  there  is  little  likelihood  of  ore  being  found  here 
similar  to  that  at  Tower. 

There  is  given  also  a  brief  account  of  a  study  of  Pokegama  Falls  and 
of  the  country  to  the  eastward  for  1 6  miles  to  Grriffin's  camp  (the  Diamond 
mine),  accompanied  by  a  sketch  map  of  this  area.  The  similarity  of  the 
iron  formation  to  that  at  the  Mailman  camp,  at  Gunflint  Lake,  and  on  the 
Penokee  range,  and  the  difi'erence  between  this  iron  formation  a".d  that  on 
the  Vermilion  range,  are  emphasized. 

The  position  of  the  Pewabic  quartzite  is  left  uncertain.  It  is  consid- 
ered, however,  to  overlie  the  Animikie  black  slate,  unless  there  are  two 
great  quartzites.  This  quartzite  has  heretofore  been  made  the  parallel  of 
the  great  quartzite  that  overlies  the  Animikie  unconformably,  but  it  is 
possible  that  it  runs  below  it  conformably.  The  great  gabbro  of  the 
Cupriferous  formation  is  regarded  as  lying  below  the  Animikie,  among 
other  reasons,  because  it  lies  next  to  and  immediately  south  of  the  gneiss 
of  the  Giant  range  without  the  appearance  of  any  black  slate  between 
them,  and  because  bowlders  of  characteristic  gabbro,  red  syenite,  and 
quartz-porphyry  occur  abundantly  in  the  later  traps  of  the  Cupriferous. 

Winchell,  N.  H.,  and  Winchell,  H.  V.  Iron  ores  of  Minnesota.  Geological 
and  Natural  History  Survey  of  Minnesota,  Bulletin  No.  6,  1891,  pages  430.  With 
a  geological  map,  26  figures,  and  il:  plates. 

This  report  was  in  part  written  before  the  discovery  of  ore  in  quantity 
in  the  Mesabi  district,  but  was  not  published  until  afterwards.  A  notice 
of  the  discovery  of  ore  is  included. 


40  THE  MESABI  IRON-BEARING  DISTRICT. 

The  report  contains  a  general  account  of  the  structural  relations  of 
the  iron  ores  of  the  Mesabi  and  Vermilion  ranges.  The  Mesabi  iron  ores 
occur  in  the  Taconic  or  Huronian,  consisting  chiefly  of  carbonaceous  and 
argillaceous  slates,  but  often  there  are  siliceous  slates,  fine-grained  quartzites, 
and  gray  limestones.  Near  the  bottom  of  the  formation  is  a  fragmental 
quartz  sandstone  having  an  apparent  thickness  of  300  feet,  which  has  been 
called  the  Pewabic  quartzite.  All  of  these  fragmentals  are  intermingled 
with  eruptive  material.  Near  Gunflint  Lake  carbonates  about  20  feet  in 
thickness  occur  near  the  bottom  of  the  Taconic.     The  authors  say: 

The  Taconic  formation  embraces  a  variety  of  ores — nontitanic  magnetites  at  the 
bottom,  jaspilitic  hematites  next  above,  soft  hematites  and  titanic  magnetites.  These 
are  found  to  constitute  a  well-marked  belt  extending  from  Pigeon  River  westward 
to  the  Mississippi  River,  although  the  titanic  magnetites  seem  to  diverge  from  this 
course  and  to  run  below  the  St.  Louis  River  a  few  miles  west  from  Duluth.  Except 
the  titanic  magnetite  of  the  gabbro,  which  is  a  primary  constituent  of  the  rock  and 
is  of  eruptive  origin,  all  the  ores  of  the  Taconic  seem  to  be  of  chemical  origin, 
and  all,  except  those  referable  to  concentration  from  oxidized  carbonates,  are  due 
to  chemical  precipitation,  as  hydrated  sesquioxides  in  the  Taconic  ocean  under 
circumstances  identical  with  those  of  the  precipitation  of  the  Keewatin  hematites. 

On  the  accompanying  geological  map  the  Laurentian,  Keewatin, 
Pewabic  quartzite,  and  Animikie  strata  are  differentiated  and  h'on  indica- 
tions are  marked  in  red.  Comparison  of  this  map  with  that  published  in 
connection  with  Volume  IV  of  the  Minnesota  survey  (see  pp.  50-52) 
shows  it  to  be  very  crude,  but  it  was  far  in  advance  of  any  previous 
map.  The  Mesabi  succession  is  also  indicated  in  a  general  cross  section  of 
northeastern  Minnesota. 

Descriptions  are  given  of  explorations  on  the  Mesabi  range,  including 
the  Stone  mine  at  Mesaba  (Mailman's  original  workings),  the  Mailman  mine 
proper,  in  sec.  11,  T.  59  N.,  R.  14  W.,  and  the  Diamond  mine,  in  sec.  15, 
T.  56  N.,  R.  24  W.,and  the  discovery  of  ore  at  the  Mountain  Iron  mine  by 
the  Merritt  brothers  is  chronicled. 

One  of  the  most  interesting  features  of  this  report  is  a  prediction  as  to 
the  future  of  the  Mesabi  range.  "The  Mesabi  ores  are  destined  to  play  a 
very  important  part  in  the  future  development  of  the  iron  industry  of  the 
State"  (p.  112).  Later  in  the  same  report,  just  after  intimation  had  been 
received  of  the  first  discovery  of  ore  on  the  range,  it  is  said,  "There  can  be 
no   reasonable   doubt     *     *     *     there    will  yet  be  mined  in  the  Mesabi 


SUMMARIES  OF  LITERATURE.  41 

range  even  greater  quantities  of  hematite  than  have  been  taken  from  that 
marvel  of  mining  districts,  the  Penokee-Gogebic  range    *    *    *"    (p.  160). 

1892. 

Van  Hise,  C.  R.  Correlation  Pajjers — Archean  and  Algonkian.  Bull.  U.  S. 
Geol.  Survey  No.  86,  1892. 

In  his  correlation  bulletin  Van  Hise  refers  the  Animikie  series  of  the 
Canadian  boundary  (the  eastward  continuation  of  the  Upper  Mesabi  series) 
to  the  Upper  Huronion,  a  part  of.  the  Algonkian  system,  and  correlates  it 
with  the  Animikie  and  the  Keewatin  series  of  western  Ontario,  the  Upper 
Vermilion  series  of  the  Vermilion  district  of  Minnesota,  the  Upper  Mar- 
quette series  of  Michigan,  the  western  Menominee  series,  the  upper  series 
of  the  Penokee-Gogebic  district,  the  Chippewa,  and  Baraboo,  Minnesota, 
and  Dakota  quartzites. 

1893. 

WiNCHELL,  H.  V.  The  Mesabi  iron  range.  Geological  and  Natural  History 
Survey  of  Minnesota,  Twentieth  Annual  Report,  1893,  pages  111-180.  See  also 
Trans.  Am.  Inst.  Min.  Eng.,  Vol.  XXI,  1893,  pages  611-686. 

Mr.  H.  V.  Winchell  here  gives  the  most  comprehensive  discussion  of 
the  Mesabi  iron  range  yet  published,  particularly  in  its  western  portion. 
It  is  also  the  first  report  written  subsequent  to  the  discovery  of  ore  on  the 
range. 

.  The  succession  of  the  Mesabi  in  descending  order  is : 

1.  Gabbro  unconf ormably  on  all  the  following Taconic 

2.  Black  slates,  Animikie _ Taconic 

3.  Greenish  siliceous  slates  and  cherts Taconic 

4.  Iron  ore  and  taconyte  horizon Taconic 

5.  Quartzite  unconformable  on  6  and  7 Taconic 

6.  Green  schists  of  the  Keewatin Archean 

7.  Granite  or  syenite  of  the  Giants  range : .  .Archean 

The  granite  of  the  Giants  range  is  bounded  on  the  north  by  a  belt  of 
crystalline  mica-schists  and  hornblende-schists,  and  on  the  south  seems  to 
have  a  direct  transition  into  the  green  schists  of  the  Keewatin.  The  green 
schist  has  a  nearly  vertical  cleavage.  The  schists  do  not  always  follow  the 
course  of  the  granite  range.  They  are  unconformably  covered  in  many 
places  by  the  quartzite.     The  quartzite  never  has  a  high  dip.     Near  the 


42  THE  MESABI  IRON-BEARING  DISTRICT. 

base  It  contains  pebbles  of  quartz  and  granite,  as  well  as  jasper  and  gi-een- 
stone.  This  quartzite  is  correlated  with  the  Pewabic  quartzite  of  Gun- 
flint  Lake,  the  Pokegama  quartzite  of  the  Mississippi  River,  that  of  Sioux 
Falls,  S.  Dak.,  and  that  of  Baraboo,  Wis.  Conformable  with  the  quartzite 
is  the  iron  ore  and  taconyte  horizon,  the  strata  of  which  are  siliceous  and 
calcareous,  and  are  banded  with  oxide  of  iron  in  beds  of  variable  length 
and  thickness.  The  ore  is  sometimes  magnetite  and  sometimes  hematite. 
To  the  banded  jaspery  chert  associated  with  the  ore  the  term  taconyte  is 
applied.  The  greenish  siliceous  slates  or  cherts  constitute  a  transition  stage 
between  the  rocks  of  the  iron  horizon  and  the  black  slates.  There  is  also  a 
considerable  mixture  of  greenish  material,  apparently  of  eruptive  origin. 
The  greater  part  of  the  rock  is  a  red,  yellow,  black,  white,  or  green  chert, 
sometimes  having  a  thickness  of  200  or  300  feet.  It  often  has  a  peculiar 
brecciated  appearance,  having  been  shattered  into  angular  fragments,  and 
recemented  by  the  same  amorphous  silica.  The  same  fracturing  is  also 
visible  in  the  iron  ore.  The  siliceous  slates  and  cherts  pass  upward  into  a 
carbonaceous  argillite  of  great  thickness,  having  a  dip  varying  from  the 
horizontal  to  20  degrees  to  the  south  or  southwest.  Locally  the  dip  is  as 
high  as  45  degrees,  in  which  case  the  ore  deposits  lie  close  to  the  green 
schists.  The  gabbro  flow  is  over  all  of  the  previous  strata.  The  effect  of 
the  heat  on  the  molten  gabbro  was  to  make  the  iron  ore  which  ah-eady 
existed  in  the  rocks  hard  and  magnetic,  although  the  magnetite  in  the  rocks 
westward  from  Mountain  Iron  mine  was  probably  too  far  from  the  gabbro  to 
have  developed  in  this  way.  There  is  good  reason  to  believe  that  the  iron 
ore  deposits  in  their  present  condition  have  been  principally  formed  since 
the  gabbro  overflow.  The  ore  deposits  occur  as  regular  beds,  which  lie  in 
almost  their  original  positions,  usually  ha^dng  a  dip  of  less  than  30  degrees 
and  passing  into  the  jaspery  quartzite  or  taconyte  in  three  directions,  and 
occasionally  on  all  sides.  The  theory  of  Irving  as  to  the  origin  of  the 
Grogebic  ores  is  partially  adopted.  The  quartzite  is  impervious  to  surface 
infiltration.  The  ore  is  regarded  as  produced  by  chemical  replacement  of 
some  mineral,  chiefly  silica,  by  oxide  of  iron.  As  evidence  of  this,  all  stages 
of  the  process  may  be  seen.  Iron  carbonate  is  found  in  the  Mesabi  rocks, 
but  it  does  not  appear  in  sufficient  quautit}-  to  permit  th-e  assumption  that 
the  source  of  the  ore  was  originally  a  carbonate.  The  solvent  for  the  silica 
was  ])robably  carbon  dioxide,  and  its  source  may  have  been  the  atmosphere, 
the   l)lack   slates,  recently  decaying  vegetation,  or  the  ore  deposits  higher 


SUMMARIES  OF  LITERATURE.  43 

up  the  slojie.  The  sihca  removed  from  the  location  of  the- iron  ores  has  been 
added  to  the  grains  of  quartz  in  the  quartzite,  has  been  deposited  as 
chalcedonic  and  flinty  sihca,  and  has  been  deposited  in  cracks  and  fissures 
in  the  slate,  which  lies  at  a  lower  elevation,  but  stratigraphically  above  the 
ore.  The  source  of  the  iron  is  believed  to  have  been  chemical  and 
mechanical  oceanic  deposits,  which  have  simply  concentrated  in  the  present 
situation,  perhaps  from  rocks  now  completely  removed  by  erosion.  The 
water  which  brought  in  the  iron  ore  to  supply  the  place  of  the  silica  taken 
away  in  solution  followed  the  natural  drainage 'courses,  surface  and  under- 
ground. The  Giants  range  is  regarded  as  having  been  uplifted  at  the  time 
of  the  gabbro  outflows,  and  to  have  been  caused  by  them. 

Brief  descriptions  are  given  of  the  following  mines :  Biwabik,  Cincinnati, 
Canton,  Kanawha  and  Hale,  Missabe  Mountain,  Ohio,  Lake  Superior,  New 
England,  Virginia,  Paddock's,  Lone  Jack,  Wyoming-,  Security,  Great 
Western,  Rouchleau,  McKinley,  and  others. 

The  general  economic  features  of  Mesabi  iron  mining  are  discussed, 
such  as  method  and  cost  of  mining,  quantity  of  ore,  transportation,  value 
to  the  State,  etc. 

WiNCHELL,  N.    H.     The   crystalline   rocks.     Geological  and  Natural  History 
Survej^  of  Minnesota.  Twentieth  Annual  Report,  1893,  pages  1-28. 

Professor  Winchell  discusses  tlie  general  age  of  the  crystalline  rocks 
of  northeastern  Minnesota. 

The  Animikie  series  lie  beneath  the  Keweenawan  and  above  the 
Keewatin  rocks.  The  Animikie  and  the  Keweenawan  together  constitute 
the  Taconic  or  Lower  Cambrian.  This  series  is  characterized  by  a  great 
quartzite  associated  with  the  iron  ores  and  cherts.  The  quartzite  (Pewabic) 
lies  unconformably  on  all  the  older  rocks.  It  often  is  conglomeratic, 
bearing  ddbris  of  the  underlying  formations.  Within  it  are  mingled 
volcanic  tuff's  from  contemporaneous  eruptions.  The  Pewabic  quartzite 
includes  that  of  Pokegama  Falls,  on  the  Mississippi  River,  and  of  Pipestone 
County.  In  the  vicinity  of  contemporaneous  volcanic  disturbances  its 
grain  is  fine,  like  jaspilite,  and  sometimes  it  has  acquired  a  dense  crystalline 
structure  from  contact  with  the  gabbro. 

Bayley,  W.  S.  Actinolite-magnetite  schists  from  the  Mesabi  iron  range  in 
northeastern  Minnesota.  Am.  Jour.  Sci.,  3d  sei'ies,  Vol.  XLVI,  1893,  pages 
176-180. 


44  THE  MESABI  IRON-BEARING  DISTRICT. 

The  actinolite-magiietite-schists  from  the  iron-bearing  formation  in  the 
vicinity  of  Birch  Lake  are  here  described,  and  attention  is  called  to  their 
similarity  to  the  actinolite-magnetite-schists  of  the  Penokee-Gogebic  district, 
described  by  Van  Hise  and  Irving. 

WiNCHELL,  N.  H.  Some  problenas  of  the  Mesabi  iron  ore.  Geological  and 
Natural  History  Sui'vey  of  Minnesota,  Twenty -first  Annual  Report,  1893,  pages 
134-143.     See  also  Am.  Geol.,  Vol.  X,  1892,  pages  169-179. 

There  is  here  given  a  general  account  of  the  rock  succession  and 
occuri'ence  of  iron  ore  in  the  Mesabi  range  and  a  discussion  of  the  origin 
of  the  iron  ore.     It  is  concluded: 

First.  The  Mesabi  ore  is  not  satisfactorilj'  explained  by  any  theory  that  has 
■yet  been  proposed  for  it,  or  for  its  equivalent  (Gogebic)  ore  on  the  south  side  of  the 
great  lake.  There  are  some  facts  that  favor  all  of  the  theories  that  have  been  pro- 
posed, but  they  meet  with  opposing  facts  of  greater  import. 

Second.  There  is  but  one  known  cause  acting  with  sufficient  force,  and  on  a 
geographic  area  sufficiently  wide,  to  which  we  can  appeal  for  the  geographic  and 
stratigraphic  distribution  of  this  ore — and  that  is  oceanic  sedimentation.  That  there 
has  been  a  profound  change  in  the  sediments  since  their  origination  is  quite  evident; 
but  whether  this  change  took  place,  in  whole  or  in  part,  prior  to  consolidation  or 
after  it  is  as  yet  unknown;  and  if  after  consolidation  it  is  equally  unknown  whether 
it  was  accomplished  in  Taconic  or  in  Recent  time.  There  seems  to  have  been  some- 
thing peculiar  either  in  the  natui'e  of  the  sediments  of  this  horizon  or  in  the  intiu- 
ences  to  which  they  have  been  subjected,  and  this  peculiarity  is  expressed  on  both 
sides  of  the  Lake  Superior  basin. 

WiNCHELL,  N.  H.  Field  observations  in  1892.  Geological  and  Natural  History 
Survey  of  Minnesota,  Twenty-first  Annual  Report,  1893,  pages  79-134. 

Here  is  a  brief  description  of  the  following-  mines:  Hale,  Cincinnati, 
Biwabik,  McKinley,  Missabe  Mountain,  Security,  Virginia;  the  exploration 
of  the  Mesaba  Syndicate  Company  in  sec.  27,  T.  60  N.,  R.  13  W. 

Mention  is  made  of  a  green  dike  in  the  iron  formation  in  the  NW.  \ 
of  NE.  \  sec.  32,  T.  60  N.,  R.  13  W. 

The  magnetic  and  siliceous  nature  of  the  ore  in  the  eastern  part  of  the 
range  is  again  emphasized,  and  attention  is  called  to  the  fact  that  tliis  part 
of  the  range  is  not  likely  to  be  productive. 

189-i. 

Elftman,  A.  H.  Preliminary  report  of  field  work  during  1893  in  northeastern 
Minnesota.  Geological  and  Natural  History  Survey  of  Minnesota,  Twenty-second 
Annual  Report,  1894,  pages  159-169. 


SUMMARIES  OF  LITERATURE.  45 

Mr.  Elftman  gives  a  detailed  petrographical  description  of  the 
actiuolite-magnetite-schists  of  the  iron  formation  on  Birch  Lake  and  south- 
westward  through  T.  60  N.,  R.  12  W.  The  description  is  accompanied 
by  a  geological  sketch  map  of  the  area.  In  approaching  the  gabbro 
contact,  augite  and  olivine  appear  intimately  associated  with  the  actinolite 
and  magnetite  of  the  Animikie  schists.  The  black  slates,  also,  in  the 
proximity  of  the  gabbro,  are  changed  to  quartz-biotite-schists. 

The  Pewabic  quartzite  at  the  bottom  of  the  Animikie  is  not  found  east 
of  Iron  Lake.     The  black  slates  are  not  found  east  of  the  Dunka  River. 

Outcrops  of  the  Animikie  below  and  inclosed  by  the  gabbro  between 
Birch  Lake  and  Akeley  Lake  have  the  same  lithological  characters  and 
composition  as  the  actinolite-schists  at  Birch  Lake,  and  are  therefore 
included  in  the  Animikie. 

The  actinolite-magnetite -schists  correspond  very  closely  in  petro- 
graphical character  and  origin  to  those  in  the  Penokee-Gogebic  series 
described  by  Van  Hise  and  Ir^-ing. 

Spurr,  J.  E.  Preliminar}'  report  of  field  work  done  in  1893.  Geological  and 
Natural  History  Survey  of  Minnesota,  Twenty-second  Annual  Report,  ISOi,  images 
115-124. 

Mr.  SpuiT  describes  the  general  features  of  the  Mesabi  range.  The 
Giants  range  granite  (Laurentian)  is  found  to  be  intrusive  in  the  Keewatin 
schists.  The  Keewatin  schists  vary  in  origin,  some  being  undoubtedly 
igneous  and  some  detrital.  Near  the  contact  with  the  granite  they  have 
been  metamorphosed  into  hornblende  and  mica-schists,  which  have  hereto- 
fore been  called  Coutchiching  or  Vermilion,  and  assigned  to  a  lower  horizon 
than  the  Keewatin.  The  Animikie  rocks  lie  unconformably  ujjon  the 
Keewatin  schists. 

The  succession  and  sti'ucture  are  represented  in  a  north-south  section 
across  the  Mesabi  range,  passing  tlirough  the  town  of  Mountain  Iron. 

Spttrr,  J.  E.  The  iron-bearing  rocks  of  the  Mesabi  range  in  Minnesota.  Geo- 
logical and  Natural  History  Surve}'  of  Minnesota.  Bulletin  No.  10. 1894,  pages  1-268. 
With  3  colored  maps  and  8  plates.     See  also  Am.  Geol.,  Vol.  XIII,  1894,  pp.  335-346. 

Mr.  Spun-  was  the  first  to  make  a  systematic  study  of  the  origin  of  the 
ores,  and  his  conclusions,  published  in  this  report,  are  of  much  interest. 


46  THE  MESABl  IRON-BEARING  DISTRICT. 

The  oldest  formation  of  the  district  is  the  Keewatin,  the  most  common 
rock  of  which  is  green  schist,  but  associated  with  this,  especially  near  the 
granites,  are  hornblende-schists  and  mica-schists.  ■  The  schists  have  a 
regional  cleavage,  which  is  nearly  uniform  in  trend,  about  north  70  degrees 
east  and  nearly  vertical.  Next  in  age  to  the  Keewatin  schists  is  the 
hornblende-granite  of  the  Giants  range.  This  range  has  an  average 
width  of  about  10  miles,  and  its  direction  is  the  same  as  that  of  the 
schistosity  of  the  Keewatin  rocks.  The  granite  is  intrusive  in  the  schists,  ^ 
as  shown  by  numerous  fragments  embedded  in  it,  by  stringers  of  the 
granite  in  the  schists,  and  by  the  metamorphism  of  the  schists  adjacent  to 
the  granite. 

Unconformably  upon  the  Keewatin  and  granitic  rocks  is  the  Animikie 
series.  The  Animikie  series  has  no  marked  folding,  slaty  cleavage,  or  schis- 
tose structure.  The  rocks  of  the  series  are  in  a  gentle  southern  monocline, 
dipping  perhaps  10  or  15  degrees  east  of  south.  This  monocline  has  gentle 
vmdulations,  with  axes  parallel  to  its  dip,  and  in  the  Virginia  area  has  been 
faulted.  The  amount  of  disturbance  is  greater  adjacent  to  the  central  part 
of  the  district,  where  are  found  the  Keweenawan  rocks.  It  is  probable  that 
the  weight  of  the  Keweenawan  rocks  has  produced  a  sinking  in  the  area 
south  of  the  Animikie,  and  that  this  has  produced  the  tilting.  The  Anim- 
ikie series  may  be  divided  into  three  chief  members — the  Pewabic  quartz- 
ite,  the  iron-bearing  member,  and  the  upper  slates.  The  Pewabic  quartzite 
is  a  fragmental  rock,  indurated  by  the  enlargement  of  quartz  grains.  It 
occasionally  passes  into  a  fine-grained  conglomerate.  The  iron-bearing 
member  is  composed  of  peculiar  rocks,  presenting  no  resemblance  to  the 
Pewabic  quartzite  or  to  the  upper  slate.  The  upper  slates  are  of  great 
thickness,  and  have  at  their  base  an  impure  limestone,  often  dolomitized  or 
sideritized. 

The  part  of  the  iron-bearing  member  from  Pokegama  Falls  to  Embarass 
Lake  is  called  the  western  Mesabi  range,  that  from  Embarass  Lake  to 
Gunflint  Lake,  the  eastern  Mesabi  range,  and  from  Gunflint  Lake  east,  the 
international  boundary  area.  The  description  of  the  iron-bearing  member 
beloAv  applies  to  the  western  part  of  the  district.  The  ii'on  formation  has  a 
thickness  varying  from  500  to  1,000  feet,  with  an  average  of  about  800 
feet.  The  dip  varies  from  less  than  10  to  as  much  as  30  degrees,  the  aver- 
age being  10  degrees.     The  width  of  the  formation  varies  correspondingly 


SUMMARIES  OF  LITEKATURE.  47 

from  2  or  3  miles  to  less  than  half  a  mile,  the  average  width  being  1  mile. 
Resting  upon  the  iron-bearing  member  is  a  great  thickness  of  fine-grained 
slates,  at  the  base  of  which  is  locally  an  impm-e  dolomitic  limestone.  When 
this  limestone  is  present  the  contact  between  the  iron-bearing  member  and 
the  upper  sla.te  can  not  be  distinctly  located. 

The  least  altered  phase  of  the  iron-bearing-  member  is  a  rock  called 
taconyte,  which  consists  of  a  background  of  cryptocrystalline,  phenocrys- 
talline,  and  chalcedonic  silica,  in  which  are  numerous  granules.  These  are 
composed  of  glauconite,  siderite,  hematite,  magnetite,  limonite,  and  crypto- 
crystalline silica,  in  the  very  freshest  phase,  the  two  former  being  predomi- 
nant. One  of  these  fresher  phases  showed,  by  analyses,  about  35  per  cent 
of  siderite  and  65  per  cent  of  glauconite. 

In  terms  of  percentage  of  the  entire  rock  the  glauconite  contains  the 
following  bases: 

Per  cent. 

Alumina  (AlA) 1-35 

Sesquioxide  of  iron  (FejOj) 1. 96 

Protoxide  of  iron  (FeO) 6. 49 

Lime  (CaO) 63 

Magnesia  ( MgO ) 92 

Water  (H,0) 62 

Soda  (Na,b) 11 

Potash  (K,0) 10 

Total 12.18 

Since  silica  can  not  be  separated  from  the  free  silica  in  the  rock,  its 
percentage  is  not  known,  but  assigning  the  percentage  of  50,  which  is  the 
usual  content  of  silica  in  glauconite,  the  composition  of  the  green  granules 
of  the  Mesabi  iron  formation,  supposedly  glauconite,  is  as  follows: 

Per  cent. 

Silica  (SiO,) 50.00 

Alumina  ( AI2O3) 5.  o-l 

Sesquioxide  of  iron  (FejOs) S.  05 

Protoxide  of  iron  (FeO) - 26.  06 

Lime  (CaO) 2.  .59 

Magnesia  (MgO) 3.78 

Soda  (Na.,0).... 45 

Potash  (K2O) .41 

Water  (H^O) 2.54 

Total 99.92 

In  the  freshest  phase  were  seen,  in  thin  section,  probably  detrital 
original  grains  of  carbonate,   recognized  by  their  cleavage  as  calcite  or 


48  THE  MESABI  IRON-BEARING  DISTRICT. 

dolomite.  From  the  taconyte,  by  a  complicated  series  of  metasomatic 
changes,  there  have  developed  cherts  and  jaspers,  which  are  sideritic, 
hematitic,  magnetitic,  or  actiuolitic,  or  two  or  more  of  these  combined. 
During  the  process  the  chert  and  iron  oxides  were  largely  concentrated  in 
alternating  bands.  The  cherts  and  jaspers  are  frequently  concretionary 
and  brecciated.  They  have  often  a  prismatic  jointing  and  horizontal 
parting. 

These  transformations  were  caused  hj  downward-percolating  waters, 
carrying  as  the  chief  agents  oxygen  and  carbonic  acid,  and  as  subordinate 
agents  sulphuric  acid  and  alkalies.  In  the  changes  from  glauconite  and 
siderite  to  the  oxides,  there  was  an  important  shrinkage  of  the  mass,  and 
this  has  resulted  in  the  brecciation,  prismatic  jointing,  horizontal  parting, 
and  banding.  The  prismatic  jointing  is  analogous  in  its  formation  to  the 
shrinkage  of  basaltic  columns  of  lava.  The  horizontal  parting  is  caused 
by  a  later  shrinkage  along  the  least  diameters  of  the  columns  formed  hj 
the  prismatic  jointing.  The  banding  is  due  to  the  removal  of  silica  and 
the  entrance  of  iron  along  the  parting. 

The  ore  deposits  rest  upon  the  Pewabic  quartzite,  or  upon  the  hard 
and  little  altered  iron-bearing  rock,  in  areas  of  especial  weakness  or  dis- 
turbance, as  (1)  actual  fault  lines,  (2)  incipient  fault  lines,  (3)  apices  of 
anticlinal  folds  and  the  troughs  of  synclines.  These  are  places  of  fracture, 
and  Avhere  abundant  waters  were  converged  often  form  wide  areas,  and 
therefore  places  where  large  quantities  of  iron  were  supplied.  The  down- 
ward-percolating water,  taking  glauconite  or  iron  carbonate  in  solution,  pre- 
cipitated the  iron  as  oxide  in  those  places  where  there  was  an  abundance  of 
oxygen,  and  at  the  same  time  took  the  silica,  in  solution,  thus  forming  the 
ore  bodies.  Between  those  of  the  largest  size  and  the  small  local  concen- 
trations there  are  all  gradations.  The  larger  deposits  of  ore  occur  where 
they  are  protected  from  glacial  erosion  on  the  north  by  a  hard  ridge  of  the 
Keewatin  rocks,  especially  when  the  hard  rocks  give  slight  elevations  on 
either  side,  so  as  to  present  a  basin-like  depression. 

The  fflauconite  in  origin  is  believed  to  be  the  same  as  modern  glauco- 
nites;  that  is,  it  has  developed  within  foraminifera  and  other  minute  shells, 
as  a  result  of  a  reaction  between  the  organic  matter  within  the  shells  and 
fine  ferriferous  clay  As  the  formation  contains  only  a  small  quantity  of 
i)i-dinary  fragmental  quartz  grains,  it  formed  in  water  at  a  depth  beyond 


SUMMARIES  OF  LITERATURE.  49 

which  much  of  these  materials  was  deposited.  As  its  upper  horizon  g-rades 
into  Hmestone,  this  indicates  a  further  subsidence  of  the  area,  so  that  the 
distance  from  the  shore  Hne  became  so  great  that  very  httle  mechanical 
detritus  was  furnished,  and  the  deposit  was  made  up  of  calcareous  matter. 

In  the  eastern  Mesabi  district  the  Animikie  strata  are  pierced  and 
intermingled  with  the  northern  border  of  the  Keweenawan  rocks,  so  that 
their  normal  attitude  is  often  much  disturbed.  With  this  change  the  iron 
of  the  iron-bearing  member  becomes  largely  magnetic  and  the  silica  hard 
and  crystalline.  It  is  concluded  that  the  iron  before  Keweenawan  time 
was  here  in  the  state  of  sesquioxide,  and  that  the  heat  of  the  igneous 
Keweenawan  rocks  and  the  disturbances  of  the  Animikie  series  produced 
by  them  are  the  causes  of  the  change  of  the  sesquioxide  of  iron  to  its  mag- 
netic form.  Thus  the  normal  process  of  decomposition  and  concentration 
was  brought  to  a  close,  and  this  probably  explains  the  poverty  of  this  part 
of  the  district  in  large  ore  deposits. 

At  the  base  of  the  Cretaceous  are  ferriferous  detrital  deposits  derived 
from  the  Animikie.  A  study  of  these  indicates  that  the  metasomatic 
processes  had  gone  far  before  Cretaceous  time,  although  they  have  since 
continued  to  the  present  time. 

Upham,  Waeren.  Preliminarj'  report  of  field  work  during^  1893  in  northeastern 
Minnesota,  chiefly  relating-  to  the  glacial  drift.  Geological  and  Natural  History 
Survey  of  Minnesota,  Twenty-second  Annnal  Report,  1894,  pages  18-66. 

Mr.  Upham  here  gives  a  general  discussion  of  the  general  geology  of 
northern  Minnesota.  The  area  of  the  Mesabi  range  between  Hibbing  and 
the  west  end  of  the  range  is  shown  to  be  occupied  by  the  Tenth  or  Itasca 
moraine;  between  Hibbing  and  Embarrass  River,  by  moraiual  material 
representing  the  merging  of  the  Itasca  moraine  and  the  Mesabi  or  Eleventh 
moraine;  eastward  from  Embarrass  River  to  Birch  Lake,  by  the  Mesabi 
moraine. 

1893-1895. 

Baylet,  W.  S.  The  basic  massive  rocks  of  the  Lake  Superior  region.  Jour. 
GeoL,  Vol.  1,  1893,  pp.  133-456,  .587-596,  688-716;  Vol.  II,  1891,  pp.  811-825;  Vol. 
Ill,  1896,  pp.  1-20. 

A  detailed  petrographic  description  of  the  gabbro  and  related  rocks  of 
northeastern  Minnesota. 

MON  XLIII — 03 4 


50  THE  MESABI  IRON-BEARING  DISTRICT. 


1896. 


Van    Hise,    C.    R.     Principles   of    North   American    pre-Cambrian    geology. 
Sixteenth  Ann.  Rept.  U.  S.  Geol.  Survey,  Ft.  I,  1896,  page  790. 

The  upper  series  of  Mesabi  and  its  eastern  equivalent,  the  Animikie, 
are  con-elated  with  the  Upper  Huronian  division  of  the  Algonkian  (as  in 
Bull.  86),  and  is  placed  as  equivalent  to  the  Animikie  and  the  Keewatin 
series  of  western  Ontario,  the  Upper  Vermilion  series  of  the  Vermilion 
district  of  Minnesota,  the  Upper  Marquette  series  of  Michigan,  the  western 
Menominee  series,  the  Upper  series  of  the  Penokee-Gogebic  district,  the 
Chippewa,  the  Baraboo,  Minnesota,  and  Dakota  quartzites,  the  Wisconsin 
Valley  series,  the  Upper  Felch  Mountain  series,  and  the  St.  Louis  slates  of 
Minnesota. 

X898. 

Elftman,  a.  H.  Geology  of  the  Keweenawan  area  in  northeastern  Minnesota. 
Am.  Geol.,  Vol.  XXI,  1898,  pp.  90-109,  175-188;  Vol.  XXII,  1898,  pp.  131-149. 

This  report  includes  a  detailed  description  of  the  gabbro  and  a  brief 
account  of  the  contact  effect  of  the  gabbro  on  the  underlying  rocks.  The 
glacial  history  of  the  northern  part  of  the  State  is  reviewed,  but  no  featui-es 
are  given  in  addition  to  those  previously  published  by  Upham. 

1899. 

WiNCHELL,  N.  H.,  and  Grant,  U.  S.  The  geology  of  Minnesota.  Geological 
and  Natural  History  Survey  of  Minnesota,  Final  Report,  Vol.  IV,  1899.  Illus- 
trated by  31  colored  geological  plates,  48  plates  of  photographic  views,  and  114 
figures. 

This  volume  is  accompanied  by  detail  maps  of  the  Mesabi  district, 
a  general  map  of  the  district,  and  maps  of  the  counties  in  which  the  Mesabi 
district  occurs.  The  county  maps  and  the  detail  maps  of  the  Mesabi  are 
made  subjects  of  special  chapters.  The  maps  are  far  in  advance  of  any- 
thing thus  far  published,  and  are  specially  accurate  with  reference  to  the 
u-on-bearing  formation.  While  the  main  features  of  the  geology  are  essen- 
tially the  same  as  those  given  in  previous  reports  of  the  Minnesota  sui-vey, 
there  are  a  number  of  minor  modifications  in  interpretations  of  the  geology. 

The  general  succession  for  the  area  west  of  Birch  Lake  is,  from  the 
top  dowi(i=: 


Animikie . 


A rchean . 


SUMMARIES  OF  LITERATURE.  51 

Succession  of  formations  west  of  Birch  Lake. 

„      .  /  1     •  1  J  ■c^\  /Flat  or  nndiilatina  till. 

Quaternary  (glacial  drift) i_        .     ,  .    '=' 

.  ITermmal  moraines. 

Cretaceous Shales,  clays,  and  conglomerates. 

Cabotian  (lower  division  of  Keweenawan) Gabbro. 

'Upper  slates. 

Black  slates. 

Taconyte  (iron  ore) . 

P  jkegama  quartzite. 

Granite  (post-Keewatin). 

Greenstones. 

Mica-schists  (in  part  in  Keewatin) 

Pewabic  quartzite  and  iron  ore. 

Lower  Keewatin. 

Spurr's  theory  of  the  origin  of  the  ores  from  the  alteration  of  glauconite 
grains  is  accepted. 

In  some  places  the  bottom  of  the  Animikie  is  distinctly  taconitic  and 
ferruginous,  while  in  others  it  is  distinctly  quartzitic  and  conglomeratic. 
The  Pokegama  qiiartzite  is  thus  not  a  continuous  formation  and  is  believed 
to  blend  into  the  iron-formation  strata  in  places. 

The  "quartzitic  rocks"  extending  eastward  from  Iron  Lake  near  the  east 
side  of  range  12,  and  geographically  continuous  with  the  Pokegama  quartzite 
and  iron  formation,  are  mapped  and  described  as  Pewabic  quartzite."  The 
Pewabic  quartzite  is  a  rock  in  which  giiinerite,  magnetite,  enstatite,  diallage, 
hypersthene,  olivine,  and  other  minerals  characteristic  of  the  gabbro  contact 
have  been  developed  and  is  supposed  to  have  resulted  from  the  alteration  of 
a  jaspilitic  phase  of  the  Keewatin.  The  altered  Pokegama  and  Pewabic 
quartzites  are  difficult  to  distinguish.  The  Pokegama  quartzite  usually  dips 
less  than  25  degrees,  becoming  horizontal,  and  the  Pewabic  usually  more 
than  75  degrees,  becoming  vertical.  The  Pokegama  quartzite  is  associated 
with  taconitic  iron  ore  and  the  Pewabic  with  jaspilitic.  The  former  is  not 
known  to  be  titaniferous ;  the  latter  is  usually  distinctly  titaniferous.  The 
Pokegama  quartzite  is  never  associated  with  the  peculiar  muscovadyte,  but 
the  Pewabic  is  never  without  it.  The  Pokegama  quartzite,  with  its  taconitic 
companion,  is  known  to  be  overlain  by  the  black  slates  of  the  Animikie,  and 
occurs  only  westward  from  Iron  Lake.  The  Pewabic  qiiartzite  is  overlain 
and  underlain  invariably  by  muscovadyte,  or  by  "gabbro,"  where  the  altera- 
tion was  intense,  and  occurs  only  eastward  I'rom  the  vicinity  of  Iron  Lake. 

o  The  conclusion  that  the  Pokegama  quartzite  blends  into  the  iron  formation  and  is  replaced  to 
the  east  by  the  Pewabic  quartzite  is  one  reached  by  Professor  Winchell.  Professor  Grant,  in  his  por- 
tions of  the  volume,  regards  the  quartzite  as  a  persistent  horizon  at  the  base  of  the  Animikie  series  in 
the  western  portion  of  the  district. 


52  THE  MESABI  IRON-BEARING  DISTRICT. 

The  Keewatiu  rocks  in  the  part  of  the  district  west  of  Bircli  Lake  are 
not  subdivided  in  the  mapping,  but  in  rock  cuts  along  the  railway  in  sees. 
15  and  22,  T.  58  N.,  R.  17  W.,  Dr.  U.  S.  Grrant  observed  certain  coarse 
fragmental  rocks  on  the  basis  of  which  he  suggests  a  division  of  the 
Keewatin  into  a  lower  igneous  portion  and  an  upper  fragmental  portion 
(pp.  372-374). 

The  evidence  for  faulting  in  the  Virginia  area,  given  by  Spurr,  is 
considered  inadequate. 

Eastward  from  Birch  Lake,  in  the  gabbro,  are  isolated  areas  of  banded  • 
ferruginous  quartzites  and  olivinitic  iron  ores  which  are  regarded  as  parts 
of  the  Animikie  caught  in  the  gabbro  flow.  These  are  found  in  the 
following  locations:  Just  north  of  Muskrat  Lake,  in  sec.  30,  T.  62  N., 
R.  10  "W.;  south  of  Disappointment  Lake,  in  sec.  4,  T.  63  N.,  R.  8  W.; 
northwest  corner  of  Thomas  Lake,  in  sec.  29,  T.  64  N.,  R.  7  W.;  north 
side  of  Fraser  Lake,  in  sec.  23,  T.  64  N.,  R.  7  W.;  in  sec.  20,  T.  64  N., 
R.  6  W.;  south  side  of  Grabbemichigamak  Lake,  in  sec.  1,  T.  64  N.,  R.  6  W. 
Similar  rocks  are  found  to  the  east  in  a  belt  running  through  Chubb, 
Akeley,  and  Gunflint  lakes,  but  as  these  lakes  are  in  the  area  covered  by 
the  monograph  on  the  Vermilion  district  their  geolog'y  is  not  summarized. 

The  petrography  of  the  gabbro  itself  is  briefly  described. 

1900. 

WiNCHELL,  N.  H.  Structural  geology  of  Minuesota.  Geological  and  Natural 
History  Survey  of  Minnesota,  Final  Report,  Vol.  V,  1900,  pages  1-80,  972-1000. 

This  volume  contains  Professor  Winchell's  final  conclusions  on  the 
general  geolog}^  of  nortliern  Minnesota  and  the  origin  of  the  Mesabi  iron 
ores.  While  much  of  it  does  not  concern  the  Mesabi  district,  it  is  here 
fully  summarized  in  order  to  show  Professor  Winchell's  latest  correlation  of 
the  Mesabi  geology  with  that  of  adjacent  areas.  (See  discussion  of  coirela- 
tion,  pp.  200-205.) 

The  ancient  rocks  of  northern  Minnesota  are  placed  in  two  main 
systems,  the  Archean  and  the  Taconic.  The  former  is  further  subdivided 
into  the  Upper  and  Lower  Keewatin,  separated  from  each  other  by  an 
unconformity.  The  Pewabic  quartzite  (see  above  summary  of  Vol.  IV)  also 
is  placed  with  the  Keewatin,  but  is  not  assigned  to  either  of  the  main  divi- 
sions. (Overlying  the  Archean  with  strong  unconformit}-  is  the  Taconic, 
represented    ]^y    Animikie   and   Keweenn\\'nn   rocks,   these  divisions   being 


SUMMARIES  OF  LITERATURE.  53 

supposed  to  represent  respectively  the  Lower  and  Middle  Cambrian  of 
other  parts  of  the  country.  The  Coutchiching  and  Laurentian  rocks  before 
mapped  as  separate  formations  are  now  included  within  the  Keewatin. 

Lower  Keewatin. — Tlie  Lower  Keewatiu  comprises  greenstone,  with  asso- 
ciated surface  volcanics  which  are  both  subaerial  and  subaqueous,  argyllitic 
slates,  siliceous  schists,  quartzites,  arkoses,  "  green wackes,"  iron  ores,  and 
marble. 

The  greenstone,  designated  the  Kawishiwin,  is  the  oldest  known  rock 
in  the  State,  and  is  supposed  to  represent  a  portion  of  the  original  crust  of 
the  earth.  With  its  associated  volcanic  rocks  it  occurs  in  two  main  belts. 
The  southern  belt  begins  in  the  vicinity  of  Gunflint  Lake  and  extends  west- 
ward by  way  of  Gobbemichigamma  Lake,  the  Kawishiwi  River,  and  White 
Iron  Lake,  to  Tower,  and  indefinitely  westward.  The  northern  belt  of 
greenstone  enters  the  State  from  Hunters  Island,  appearing  conspicuously 
at  the  south  side  of  Basswood  Lake.  At  Pipestone  Rapids  and  Fall  Lake 
it  widens  southward  and  apparently  unites  at  the  surface  with  the  southern 
belt,  the  overlying  Upper  Keewatin  being  absent  for  a  distance  of  a  few 
miles.  But  farther  west  it  is  again  divided  by  the  Stuntz  conglomerate,  the 
northern  arm  running  to  the  north  of  Vermilion  Lake,  west  of  which  its 
extension,  is  unknown,  and  the  southern  one  running  south  of  the  lake. 

The  fragmental  stratified  rocks  of  the  Lower  Keewatin  are  most 
important  toward  the  western  part  of  the  area  of  exposure  of  crystalline 
rocks.  They  occupy  a  wide  area,  south,  west,  and  north  of  Tower.  The 
iron  ores  of  Tower  and  Ely  on  the  Vermilion  iron  range  occur  in  the  upper 
part  of  the  Lower  Keewatin.  It  is  probable  that  the  immediately  inclosing 
rock  is  a  sedimentary  one,  although  composed  of  the  elements  of  a  basic 
eruptive.  The  sediments  extend  south  to  the  Giants  range  of  granite, 
where  they  are  metamorphosed  to  mica-schists  by  the  granite.  Toward  the 
west  they  extend  as  far  as  the  Mississippi  River  and  its  northern  tributaries 
and  across  the  Bowsti'ing,  although  the  drift  prevents  the  delimitation  of  the 
belt.  To  the  northwest  they  extend  toward  Rainy  Lake,  in  this  direction 
being  converted  into  mica-schists  and  gneisses  by  the  intrusion  of  granite; 
in  unmodified  form  they  are  found  at  one  point  only  on  Rainy  Lake. 
These  fragmental  rocks  of  the  Lower  Keewatin  doubtless  also  underlie 
most  of  the  central  and  southwestern  part  of  the  State  as  far  as  the  Minne- 
sota River.  Here  they  dip  beneath  the  later  formations  in  the  southwestern 
portion  of  the  State,  and  probably  occupy  a  wide  patch  in  South  Dakota. 


54  THE  MESABI  IRON-BEARING  DISTRICT. 

South  of  the  Giants  range  they  occur  also,  but  as  they  are  covered  by  the 
gabbro  and  Animikie  toward  the  east  and  the  drift  deposits  of  the  St.  Louis 
Valley  toward  the  west  their  geographic  boundaries  are  mostly  unknown. 
They  appear  in  the  central  and  western  portions  of  Carlton  County,  where 
their  line  of  separation  from  the  Upper  Keewatin  is  quite  obscure,  and  in 
the  central  and  western  portions  of  Morrison  County.  The  Lower  Keewatin 
marble  is  seen  at  Lake  Ogishke-Muncie  and  at  Pike  Rapids,  on  the 
Mississippi. 

The  Lower  Keewatin  was  terminated  by  a  period  of  extensive  folding 
and  intrusions  of  granite  and  basic  rocks. 

The  Pewabic  quartzite  belongs  with  the  Keewatin,  but  whether  to  the 
Lower  or  Upper  Keewatin  is  not  known.  This  formation  includes  altered 
quartzites  and  iron  ores  between  the  granite  and  gabbro  in  the  immediate 
vicinity  of  Birch  Lake  and  small  patches  of  similar  rocks  in  sec.  30, 
T.  62  N.,  R.  10  W.;  on  the  south  shore  of  Disappointment  Lake;  on  the 
north  shore  of  Eraser  Lake;  on  the  south  shore  of  Gabbemichigamma ;  at 
Akley  Lake,  forming  the  so-called  Akley  Lake  series  extending  from  the 
west  side  of  sec.  34,  T.  65  N.,  R.  5  W.,  to  the  eastern  part  of  sec.  27, 
T.  65  N.,  R.  4  W. 

Upper  Keewatin. — Tlic  Uppcr  Kccwatin  occurs  in  troughs  in  the  Lower 
Keewatin,  particularly  in  one  main  trough  the  axis  of  which  is  traceable 
from  Vermilion  Lake  to  Saganaga  Lake.  The  northern  arm  of  this  sjuicline, 
consisting  of  granites,  gneisses,  associated  mica-schists,  and  in  some  places 
earlier  greenstones,  extends  from  the  northern  part  of  Vermilion  Lake  through 
Basswood  Lake  to  the  northern  side  of  Hunters  Island.  The  southern  arm, 
consisting  of  Lower  Keewatin  green  schists  and  other  schists,  penetrated 
by  the  granite  of  the  Giants  range,  extends  from  Pokegania  Falls  on  the 
southwest  toward  the  northeast  until  cut  out  by  the  encroachment  of  the 
gabbro  from  the  south.  The  Upper  Keewatin  consists  very  largely  of  con- 
glomerates, but  also  includes  graywackes,  argyllites,  quartzites,  and  jaspi- 
lites,  in  general  coarser  than  those  of  the  Lower  Keewatin.  Volcanic  rocks 
are  less  important  than  in  the  Lower  Keewatin,  although  still  present. 
There  is  no  general  order  of  succession  in  the  Upper  Keewatin  excepting 
that  it  can  be  said  that  it  is  in  general  conglomeratic  at  the  bottom. 

After  Upper  Keewatin  time  both  the  Lower  and  Upper  Keewatin 
were  subjected  to  another  folding,  the  axis  of  which  had  a  general  paral- 
lelism with  the  earlier  folding,  with  the  result  that  the  Ui)per  Keewatin  lies 


SUMMARIES  OF  LITERATURE.  55 

in  naiTow  synclines  in  the  Lower  Keewatiu  and  in  places  is  nearly  or 
quite  vertical. 

Associated  with  the  Keewatin  rocks  are  granites  of  at  least  two  periods 
of  intrusion,  one  later  than  the  Lower  Keewatin  and  one  later  than  the 
Upper  Keewatin.  The  later  granite  is  believed  to  be  represented  by  the 
higher  parts  of  the  Giants  range  and  the  Snowbank  Lake  g-ranite.  The 
earlier  granite  is  represented  by  the  granites  at  Kekequabic  Lake,  Sag- 
anaga  Lake,  Basswood  Lake,  Burntside  Lake,  Vermilion  Lake,  Lac  la 
Croix,  and  Kabetogoma  Lake.  The  origin  of  the  granite  is  discussed  and 
the  same  conclusions  are  reached  as  in  a,  previous  article." 

The  Taconic. — This  is  uuconformably  above  the  Keewatin  rocks.  It 
comprises  the  Animikie  and  Keweenawan  divisions. 

The  Animikie  rocks  enter  the  State  at  Pigeon  Point  and  run  westward 
along  the  international  boundary  to  the  eastern  part  of  sees.  22  and  27, 
T.  65  N.,  R.  4  W.  They  reappear  again  southwestward  from  Birch  Lake 
on  the  northwest  side  of  the  gabbro  mass,  and  thence  continue  along 
the  south  side  of  the  Giants  range,  constituting  the  Mesabi  iron  series,  to 
Pokegama  Falls.  The  higher  parts  of  the  Animikie  are  best  developed 
toward  the  east,  while  the  lower  parts  are  best  developed  toward  the  west. 

The  Animikie  rocks  comprise  the  Pokegama  quartzite,  Mesabi  iron- 
bearing  formation,  and  some  limestone  and  slate,  all  strictly  conformable 
with  one  another.  The  thickness  is  several  hundred  feet,  sometimes  reach- 
ing nearly  1,000  feet.  The  dip  of  the  series  is  uniformly  to  the  south, 
8  to  12  degrees. 

The  iron-bearing  formation  and  the  Pokegama  quartzite  constitute  the 
base  of  the  formation.  The  quartzite  in  places  is  beneath  the  iron  forma- 
tion; in  other  places  it  is  in  the  same  horizon,  and  in  still  others  is  above 
the  iron  formation.  Commonly  the  base  of  the  Animikie  is  marked  by  a 
conglomerate  containing-  debris  from  the  underlying  Keewatin  rocks.  This 
is  a  narrow  horizon  which  soon  graduates  upward  into  a  quartzite  known 
as  the  Pokegama  quartzite  from  its  typical  development  near  Pokegama 
i'alls,  on  the  Mississippi  River.  The  thickness  of  the  quartzite  is  not 
known  to  exceed  50  feet  and  is  sometimes  less  than  25  feet. 

Above    the    quartzite,    or   in   alternating    beds   with  it,    or  below  it, 

«The  origin  of  the  Arehean  igneous  rocks,  bj'  N.  H.  Winchell:  Proc.  Am.  Assoc.  Adv.  Sci., 
Vol.  XLVII,  1898,  pp.  303,  304  (abstract);  also  Am  Geol.,  Vol.  XXII,  1898,  pp.  299-310;  sum- 
marized in  .Jour.  Geol.,  Vol.  VII,  1899,  p.  194. 


56  THE  MESABI  IRON-BEARING  DISTRICT. 

appears  the  iron-bearing  or  taconyte  member  of  the  Animikie,  which 
contains  the  iron-ore  deposits  of  tlie  ^Mesabi  iron  range.  The  ore  is 
nsuallv  hematite  in  tlie  western  part  of  the  range  and  magnetite  in  the 
eastern  part. 

Origin  of  ore. — Thc  Ore  WPS  previouslj  snpposed  to  have  been  derived 
from  the  alteration  of  a  greenish  glanconitic  sand  rock,  but  later  work  has 
seemed  to  show  that  the  green  sand  is  a  volcanic  sand,  and  that  the  so-called 
taconitic  rock  itself  has  resulted  from  igneous  forces.  This  is  accounted 
for  by  supposing  a  chain  of  active  volcanoes  to  have  existed  where  the 
Mesabi  iron  range  is  now  found.  These  volcanoes  yielded  flows  and 
ejectamenta  to  the  adjacent  waters,  which  have  been  modified  into  the  various 
phases  of  the  iron  formation  now  seen.  This  volcanic  epoch  may  have  a 
deep-seated  connection  with  the  Cabotian  or  lower  division  of  the  Kewee- 
nawan  (described  later). 

Above  the  iron- bearing  member  is  an- impure,  dark-colored  limestone  a 
few  feet  in  thickness,  not  exceeding  20.  It  extends  apparently  the  whole 
length  of  the  Mesabi  range,  but  has  been  identified  in  two  places  only,  sec. 
7,  T.  58  N.,  R.  17  W.,  and  doubtfully  on  the  shores  of  Gunflint  Lake. 
This  limestone  may  be  regarded  as  the  basal  horizon  of  the  next  over- 
lying rock. 

The  black  slate  is  probably  several  thousand  feet  in  thickness  and 
constitutes  the  bulk  of  the  Animikie.  In  the  neighborhood  of  Gunflint 
Lake  it  has  been  divided  by  Dr.  Grant  into  a  lower  black  slate  division 
and  an  upper  graywacke-slate  division,  both  of  which  membei's  are 
interleaved  with  diabase  sills. 

In  the  Indian  reservation  at  Grand  Portage  and  at  various  places 
along  the  Grand  Portage  trail  is  a  graywacke,  which  is  supposed  to  overlie 
the  black  slate  member,  but  its  extent  and  stratigraphical  position  have  not 
been  satisfactorily  established. 

The  top  of  the  Animikie  has  not  been  identified.  The  first  recognizable 
datum  plane  after  the  close  of  the  Animikie  is  the  Puckwunge  conglomerate, 
supposed  to  be  the  fragmental  base  of  the  Keweenawan. 

At  one  or  two  places  southwestward  from  Birch  Lake,  and  at  Little 
Falls,  on  the  Mississippi  River,  and  in  Morrison  County,  the  Animikie  has 
been  converted  into  a  mica-schist. 

The  age  of  the  Animikie  is  believed  to   be  Lower  Cauibrian  for  the 


SUMMARIES  OF  LITERATURE.  57 

following  reasons:  It  grades  upward  into  Upper  Cambrian  rocks,  as  seen 
on  the  south  side  of  Lake  Superior.  The  derivation  of  the  iron  ores  from 
a  glauconitic  green  sand  indicates  that  large  quantities  of  foraminiferal 
organisms  once  lived  in  the  Animikie  Ocean,  and  Matthew  has  shown  the 
existence  of  foraminiferal  organisms  associated  with  the  iron  ore  in  the  St. 
Johns  group  of  New  Brunswick.  Further,  the  Animikie  has  a  uniformly 
low  dip,  while  the  lower  strata  are  all  highly  tilted.  There  must  therefore 
have  been  a  great  lapse  of  time  between  the  deposition  of  the  two  series. 

The  Keweenawan. — The  Puckwuuge  couglomcrate  is  taken  to  be  the  frag- 
mental  base  of  the  Keweenawan,  although  certain  igneous  rocks  which 
antedate  it,  and  which,  perhaps,  are  contemporaneous  with  the  upper 
portions  of  the  Animikie,  are  also  called  Keweenawan.  The  conglomerate 
is  found  at  Grand  Portage  Island,  at  Isle  Royale,  on  the  Baptism  River,  at 
Little  Marais,  on  Manitou  River,  at  the  deep  well  at  Short  Line  Park  near 
Duluth,  and  at  New  Ulm. 

Above  this  conglomerate  are  conglomerates  and  sandstones  of 
Keweenawan  age  which  are  stratified  with  lavas  of  diabasic  nature.  Still 
higher  up  the  eruptive  rocks  become  less  in  quantity  and  the  fragmental 
rock  is  a  sandstone,  known  as  the  Hinckley  sandstone,  quarried  in  the 
gorge  of  the  Kettle  River  in  Pine  County.  This  in  turn  grades  up  into 
typical  Upper  Cambrian  sandstones  of  the  St.  Croix  Valley.  The  term 
Potsdam  is  restricted  to  the  Puckwuuge  conglomerate  and  the  hardened 
quartzites  immediately  overlying  it,  represented  by  the  Sioux  quartzite, 
the  Baraboo  and  Barron  County  quartzites  of  Wisconsin,  the  quartzite  at 
Grand  Portage  Island  and  west  of  Grand  Portage  village,  the  New  LTm 
quartzite  in  Cottonwood  County,  and  the  quartzite  in  Pipestone  County. 

The  igneous  rocks  of  the  Keweenawan  vary  in  age  from  late  Animikie 
time  to  the  top  of  the  Keweenawan  series.  They  are  divided  into  two 
groups,  the  Cabotian  or  Lower  Keweenawan  and  the  Manitou  or  Upper 
Keweenawan. 

The  Cabotian  division  includes  gabbro  and  contemporaneous  red  rock 
and  their  surface  lavas,  and  all  other  dikes  and  sills  which  are  associated 
with  but  are  younger  than  the  Animikie  clastic  rocks  and  which  are  older 
than  the  Puckwuuge  conglomerate.  The  lower  member  of  the  Cabotian  is 
the  gabbro,  which  covers  an  enormous  area.  It  extends  on  the  east  to 
East  Greenwood  Lake,  in  T.  64  N.,  R.  2  E.     On  the  north  it  is  bounded  by 


58  THE  MESABI  IRON-BEARING  DISTRICT. 

the  Auimikie  strata  of  the  Mesabi  iron  range.  Its  westernmost  exposure 
is  in  the  vicinity  of  Short  Line  Park,  Duluth.  The  southern  limit  is 
irregular,  swinging  from  East  Greenwood  Lake  in  a  zigzag  manner  through 
T.  63  N.,  R  1  W.;  T.  62  N.,  R.  2  W.;  T.  62  N.,  R  4  W.;  T.  60  K.,  R  6  W.; 
T.  60  N.,  R  7  W.;   T.  58  N.,  R  10  W.,  and  T.  55  N.,  R  11  W.,  to  Duluth. 

Along  the  northern  and  northwestern  side  of  the  great  gabbro  mass 
the  gabbro  is  plainly  intrusive  on  the  older  formations,  Animikie  and 
Keewatin. 

From  the  northern  border  of  the  gabbro  many  sills  offshoot  and 
penetrate  the  Animikie  strata  parallel  to  the  bedding.  These  are  known 
as  the  Logan  sills. . 

Near  its  contact  with  the  underlying  rocks,  both  the  Animikie  and  the 
Keewatin  series,  there  are  various  altered  rocks  which  can  be  connected 
in  places  with  the  gabbro  and  in  places  with  the  underlying  rocks.  To 
these  altered  rocks  the  term  "muscovadyte"  has  been  applied.  It  includes 
the  various  so-called  peripheral  phases  of  the  gabbro. 

On  the  southern  and  eastern  border  the  gabbro  is  penetrated  by  and 
penetrates  in  a  confused  manner  the  red  rock,  with  which  it  alternates  both 
structurally  and  areally.  It  is  believed  to  have  resulted  from  the  meta- 
morphism  by  the  gabbro  of  the  Animikie,  and  perhaps  earlier  fragmentals. 

As  the  granites  of  the  Archean  are  believed  to  have  resulted  from  the 
softening  of  acid  fragmentals,  so  the  gabbro  may  have  been  the  result  of 
the  metamorphism  or  re-fusion  of  the  Keewatin  greenstones. 

The  anorthosite  masses  of  the  Beaver  Bay  diabase,  supposed  by  Law- 
son  to  be  of  Archean  age  and  to  underlie  unconformably  the  Beaver  Bay 
diabase,  are  believed  to  represent  segregation  phases  in  the  main  gabbro 
flow,  and  to  be  the  same  as  anorthosite  masses  in  the  gabbro  proper  to  the 
west. 

The  Beaver  Bay  diabase  is  believed  to  represent  the  upper  portion  of 
the  great  gabbro  flow,  and  to  be  due  to  the  first  and  greatest  movement 
of  tlie  gabbro  toward  Lake  Superior.  The  Logan  sills  belong  to  this  part  of 
the  gabbro  flow. 

The  Manitou  division  of  the  Keweenawan  includes  the  surface  flows, 
sills,  and  dikes  which  accompanied  and  followed  the  Puckwunge  conglom- 
erate. These  eruptives,  with  the  elastics  associated  with  them,  do  not  have 
a  thickness  in  Minnesota  of  more  than  1,000  feet.  These  lava  sheets  extend 
along  the  shore  of  Lake  Superior  from  near  Baptism  Rixcr  to  near  Grand 


SUMMARIES  OF  LITERATURE.  59 

Marais,  except  where  replaced  at  intervals  b)^  the  Beaver  Bay  diabase  or 
some  of  the  intersheeted  fragmentals.  They  occur  also  in  the  neighbor- 
hood of  Grand  Portage  Bay,  but  their  extent  here  is  not  definitely  known. 
General. — Tho  most  important  petrological  conclusions  determined  from 
the  examination  of  the  Minnesota  crystalline  rocks  are  three  in  number: 

1.  All  the  granites  of  the  Archean  can  be  explained  on  the  assumption 
that  they  are  intrusives  representing  the  metamorphosed  conditions  of  clas- 
tic rocks  adjacent  to  the  observed  intrusions,  rendered  plastic  by  the  force 
of  dynamic  metamorphism  accompanied  by  moisture. 

2.  The  basic  Keweenawan  gabbro  and  its  derivatives  are  derived  from 
the  metamorphism  and  complete  re-fusion  of  the  Archean  greenstones  and 
their  attendants. 

3.  The  green  sand  of  the  Mesabi  iron-bearing  formation  appears  to 

have  resulted  from  a  volcanic  sand,  and  the  taconite  itself,  from  igneous 

forces. 

Grant,  U.  S.  Contact  metamorphism  of  a  basic  igneous  rock.  Bull.  Geol.  Soc. 
America,  Vol.  XI,  1900,  pages  503-510. 

Dr.  Grant  describes  the  contact  metamorphism  caused  by  the  great 
gabbro  of  northeastern  Minnesota  on  the  rocks  with  which  it  comes  in  con- 
tact. These  are  of  particular  interest  as  explaining  the  character  of  the 
iron-bearing  rocks  of  the  eastern  portion  of  the  Mesabi  range.     He  says: 

That  the  great  mass  of  gabbro  at  the  base  of  the  Keweenawan  in  Minnesota  has 
features  which  indicate  its  intrusive  rather  than  its  extrusive  nature;  that  one  of  the 
most  important  of  these  features  is  the  marked  contact  zone  along  the  lower  or  north- 
ern side  of  this  mass;  that  in  this  zone  a  complete  recr3'stallization  of  the  strata  has 
been  effected,  at  times  for  a  distance  of  a  few  hundred  feet  from  the  igneous  rock, 
with  less  pronounced  effects  extending  for  a  quarter  of  a  mile  or  more;  that  the  rocks 
resulting  from  the  contact  metamorphism  of  the  iron-bearing  member  of  the  Animikie 
are  peculiar I3'  rich  in  minerals  of  the  basic  rocks — that  is,  in  augite,  hypersthene,  and 
olivine;  that  the  materials  for  these  minerals  were  present  in  the  quartz-magnetite- 
amphibole  slates  of  the  Animikie,  and  consequenth'  that  it  is  not  necessar}'  to  con- 
sider these  minerals  as  derived  from  the  gabbro,  and  that  the  contact  effects  on  some 
altered  basic  igneous  rocks  have  been  to  reproduce  the  original  mineral  character  of 
these  rocks  and  to  produce  textures  partially  similar  to  true  igneous  rocks. 

The  petrography  of  the  gabbro  itself  is  summarized. 
The  above  discussion  is  based  primarily  on  facts  observed  eastward 
from  Birch  Lake. 


60  THE  MESABI  IRON-BEARIMG  DISTRICT. 

1901. 

WiNCHELL,  N.  H.  Geological  atlas,  with  synoptical  descriptions.  Geological 
and  Natural  Histoiy  Survej"  of  Minnesota,  Final  Report,  Vol.  VI.  1901. 

This  atlas  contains  the  maps  published  with  Volume  V  of  the  Minnesota 
survey  and  in  addition  a  geological  map  of  the  State.  The  synoptical 
descriptions  of  the  plates  contain  no  features  not  given  in  Volumes  IV  and 
V.  The  mapping  of  the  iron  formation  between  Dunka  River  and  Birch 
Lake  as  Pewabic  quartzite  is  abandoned. 

Van  Hise,  C.  R.  ,  and  Leith,  C.  K.  The  iron-ore  deposits  of  the  Lake  Superior 
region  [the  part  on  the  Mesabi  district].  Twenty -first  Ann.  Rept.  U.  S.  Geol.  Survey, 
Pt.  Ill,  1901,  pages  351-370.  Accompanied  by  a  map  of  central  portion  of  the 
district,  by  C.  K.  Leith. 

This  is  a  preliminary  report  on  the  district,  containing  a  brief  account 
of  the  essential  features,  more  fully  described  in  the  present  monograph. 
The  first  announcement  is  here  made  of  the  presence  and  distribution  of 
the  Lower  Huronian  and  Archean  series,  as  these  terms  are  used  by  the 
United  States  Geological  Survey.  Spurr's  conclusion  that  the  ores  have 
developed  from  a  hydrous  feiTous  silicate  is  confirmed,  but  the  original 
gi'een  fen-ous  silicate  granules  are  thought  not  to  be  glaucouite,  as  named 
by  Spurr. 

1902. 

Sptjer,  J.  E.  The  original  source  of  the  Lake  Superior  iron  ores.  Am.  Geol.. 
Vol.  XXIX,  1902,  pages  335-349. 

After  the  appearance  of  the  above  report  by  Van  Hise  and  Leith, 
Spurr  restated  his  position,  concluding — 

1.  That  the  iron  ores  of  the  Mesabi  range  and  the  varied  and  peculiar 
rock  types  of  the  iron-bearing  formation  are  derived  from  the  alteration 
and  rearrangement  of  a  sedimentary  rock  containing  large  quantities  of  a 
green  hydrous  ferrous  silicate,  in  generally  rounded,  small,  separate  grains. 

2.  That  the  rocks  containing  iron  carbonate,  including  the  phases 
called  cherty  siderites  and  sideritic  cherts,  are  one  of  the  results  of  alter- 
ation of  this  original  rock,  the  iron  carbonate,  and  also  a  large  proportion 
of  the  silica,  being  derived  from  the  green  silicate. 

3.  That  the  green  silicate  was  formed  large!}'  through  the  agency  of 
organic  matter. 


SUMMARIES  OF  LITERATURE.  61 

4.  That  its  habit,  form,  optical  and  chemical  qualities  mark  it  as 
belonging  to  the  class  of  glauconites,  and  mark  the  original  rock  as  a 
green  sand. 

5.  That  in  accordance  with  what  is  known  of  the  formation  of  green 
sand,  the  iron,  silica,  etc.,  of  which  the  glauconite  is  comjDosed  were 
probably  derived  largely  from  fine  land  silt;  in  pai't,  also,  from  solution  in 
sea  v-^ater. 

6.  That  the  above  conclusions  probably  apply  to  most  of  the  other 
Lake  Superior  iron  ores. 

For  a  full  discussion  of  the  literature  covering  the  eastward  continua- 
tion of  the  Mesabi  range — that  is,  the  area  in  the  neighborhood  of  Akeley 
and  Grunflint  lakes  and  eastward — the  reader  is  referred  to  Monograph  XLV, 
on  the  Vermilion  district. 

ECONOMIC  REPORTS. 

In  addition  to  the  above  reports,  dealing  mainly  with  the  geology 
of  the  district,  there  have  appeared  a  large  number  of  articles  on  ihe 
economic  features  of  the  district,  including  descriptions  of  mines,  mine 
methods,  cost,  production,  transportation,  etc.  Below  is  given  a  list  of  such 
of  these  articles  as  are  signed  that  have  come  to  our  notice.  They  are  so 
numerous  and  so  widely  scattered  in  trade  journals  that  it  is  certain  that 
some  have  been  overlooked: 

Bacon,  D.  S.  Methods  of  working  on  the  Mesabi  iron  range.  Engineering  and 
Min.  Jour. ,  Vol.  LXIV,  1897,  pp.  306-307. 

Bailey,  C.  E.  Mining  methods  on  the  Mesabi  range.  Trans.  Am.  Inst.  Min.  Engi- 
neers, Vol.  XXVII,  1897,  pp.  529-536. 

Brackenbuet,  Ctril.  Methods  of  mining  on  the  Mesabi  range.  Mines  and  Min- 
erals, Vol.  XXI,  1900,  pp.  150-152. 

Channing,  J.  Parke.  Lake  Superior  iron  ore.  The  Mineral  Industrj';  its  Statistics, 
Technology,  and  Trade  to  the  End  of  1894,  Vol.  Ill,  1895,  pp.  375-102. 

Chester,  A.  H.  The  iron  region  of  northern  Minnesota.  Geol.  and  Nat.  Hist. 
Surve.y  of  Minn..  Eleventh  Ann.  Rept.,  1881,  pp.  151-167. 

Denton,  F.  W.  Methods  of  iron  mining  in  northern  Minnesota.  Am.  Inst.  Min. 
Engineers,  Vol.  XXVII,  1897,  pp.  344-390. 

Open  pit  mining,  with  special  reference  to  the  Mesabi.     Proc.  Lake  Sup. 

Min.  Inst.,  Vol.  Ill,  1895,  pp.  84-92. 

Elftman,  a.  H.  Ore  deposits  in  Minnesota.  Yearbook  Soc.  of  Engineers,  Univ. 
Minn.,  Vol.  IV,  1896,  pp.  115-117. 


62  THE  MESABl  IRON-BEARING  DISTRICT. 

Head.  jEREinAH.  and  Head.  A.  P.     The  Lake  Superior  iron-ore  mines  and  their 

influence  vipon  the  production  of  iron  and  steel.     Proc.  Inst.  Civ.  Engineers 

(Enoland).  Vol.  CXXXVH,  1899.  pp.  72-102;  discussion  of  sand,  pp.  103-130, 

with  Plate.s  III  and  IV. 
HuLST,  N.  p.     Lake  Superior  methods  of  mining.     Proc.  Engineers  Soc.  West. 

Penn.,  Vol.  XV,  1899,  pp.  62-104. 
Lamnees,  T.  L.     Engineering  and  Min.  Jour.,  Vol.  LIV,  1892,  p.  579. 
Lo^GTEAR,  E.  J.     Explorations   on   the   Mesabi   range.     Trans.    Am.    Inst.   Min. 

Engineers,  Vol.  XXVII,  1897,  pp.  537-541. 
Warken,  O.  B.     The  Mahoning  iron  mine.     Iron  Age,  Vol.  LXIV,  1899,  pp.  1-3. 
Wedding,  H.     Stahl  und  Eisen,  Vol.   XVI,  pp.   7-13,  with  5  illustrations  taken 

from  the  Iron  Age  and  1  from  Hai'per's  Weekty. 
Wilkinson,  C.  D.     Systems  of  mining  in  Minnesota  iron  mines.     Yearbook  Soc.  of 

Engineers,  Univ.  Minn.,  Vol.  Ill,  1895,  pp.  47-51. 
AViNCHELL,  H.  V.     Methods  of  mining.     Iron  Trade  Review,  July  21,  1892. 

The  Mesabi  iron  range.     Geol.  and  Nat.  Hist.  Survey  of  Minn.,  Twentieth 

Ann.   Rept.,  1891,  Minneapolis,   1893,  pp.   111-180.     Trans.   Am.   Inst.   Min. 
Engineers,  Vol.  XXI,  1893,  pp.  644-686. 

The  iron  ranges  of  Minnesota.     Proc.  Lake  Sup.  Min.  Inst.,  Vol.  IH,  1895, 

pp.  1.5-32. 

The  Lake  Superior  iron-ore  region,  U.  S.  A.  Trans.  Fed.  Inst.  Min.  Engi- 
neers, Vol.  XIII,  1896-97,  pp.  493-562.  Accompanied  by  map  of  the  Great 
Lakes,  and  6  sections  of  the  Mesabi. 

Minnesota  iron  mining  economically  and  statistically  considered.     Geol.  and 


Nat.  Hist.  Survey  of  Minn.,  Final  Report,  Vol.  IV,  1899,  pp.  581-616. 
WixcHELL,  H.  v.,  and  Jones,  J.  T.     The  Biwabik  mine.     Trans.  Am.  Inst.  Min. 

Engineers,  Vol.  XXI,  1893,  pp.  951-961. 
WrscHELL,  N.  H.,  and  Winchell,  H.  V.     Iron  ores  of  Minnesota.     Geol.  and  Nat. 

Hist.  Survey  of  Minn.,  Bull.  No.  6,  1891. 
Winchell,  N.  H.     Geology  of  the  iron  oi-es  of  Minnesota.     Geol.  Soc.  Australasia, 

•    Vol.  I,  1892,  pp.  171-181. 
The  discovery  and  development  of  the  iron  ores  of  Minnesota.     Minn.  Hist. 

Soc.  Collections,  Vol.  VIII,  Pt.  I,  1895,  pp.  25-40,  with  geological  map. 
WooDBRiDGE,  D.  E.     Ii'on-ore  mining  on  the  Mesabi  range.     Engineering  and  Min. 

Jour..  Vol.  LVI,  1893.  p.  163. 
See  also  unsigned  article  in  Iron  Age,  Vol.  LVI,  1895,  pp.  216,  277,  and  386. 


CHAPTER  III. 

THE  BASEMENT  COMPLEX,  OR  ARCHEAN. 

DISTRIBUTIOlSr. 

The  Archean  rocks  of  tlie  Mesabi  district  are  confined  to  its  central 
portion.  They  are  found  north  and  northwest  of  Nashwauk,  northwest  of 
Hibbing;  north  and  northeast  of  Mountain  Iron;  in  the  southerly  projection 
of  the  Mesabi  range  known  as  the  "Horn,"  bounded  by  the  cities  of 
Virginia,  Eveleth,  Sparta,  and.  McKinley;  north  of  Biwabik,  and  eastward 
to  near  the  east  line  of  R.  16  W.  With  the  exception  of  the  portion  of  the 
Archean  area  east  of  Embarrass  Lake,  exposures  are  sufficiently  common  to 
allow  of  a  fairly  close  determination  of  the  boundaries.  East  of  Embarrass 
Lake  the  mapping  is  based  on  the  presence  of  abundant  Archean  fragments 
in  the  di'ift. 

Included  in  the  areas  mapped  as  Archean  north  of  Mountain  Iron 
are  several  small  patches  of  Lower  Huronian  rocks.  Exposures  are  so  few, 
they  are  so  mixed  in  the  same  exposure  with  Archean  rocks,  and  they  are 
metamorphosed  to  such  difficultly  recognizable  forms  that  their  accurate 
delimitation  on  the  general  map  is  not  possible.  Their  distribution,  so  far 
as  worked  out,  is  shown  on  a  special  large-scale  plat  (PI.  V). 

KINDS  OF  ROCKS. 

The  Archean  is  represented  by  dolerites  (and  their  altered  equivalents, 
metadolerites  or  diabases),  basalts  (and  their  altered  equivalents,  metaba- 
salts),  diorites,  peridotites(?),  micaceous,  chloritic,  and  hornblendic  schists, 
granites,  and  poi'phyritic  rhyolites.  In  abundance  the  rocks  stand  in  about 
the  following-  order:  the  micaceous,  chloritic,  and  hornblendic  schists, 
basalts,  dolerites,  porphyritic  rhyolites,  granites,  and  diorites.  The  basic 
rocks  have  commonly  a  green  color  and  are  usually  referred  to  locally  as 
greenstones  or  green  schists.  Tliey  are  so  intricately  intermingled  that 
they  are  given  one  color  on  the  general  map,  but  in  the  area  northwest 


64  THE  MESABI  IRON-BEARING  DISTRICT. 

of  Hibbing,  to  illustrate  their  complexity,  they  have  been  separately 
indicated  on  a  special  large-scale  map  (PL  IV).  The  acid  igneous  rocks, 
consisting  of  the  porphyritic  rhyolites  and  the  granites,  are  mapped  undei- 
another  color. 

All  of  these  rocks  have  their  counterparts  in  other  iron  districts  of  the 
Lake  Superior  region.  In  the  Vermilion  and  Crystal  Falls  districts,  where 
especially  well  developed,  Clements  has  described  each  phase  in  great 
detail.  On  this  account  the  following  description  of  the  rocks  of  the 
Archean  of  the  Mesabi  district  is  A-ery  brief.  The  names  used  by  Clements 
in  the  Crystal  Falls  and  Vei'milion  districts  are  applied  throughout.  In 
case  the  reader  desires  to  know  moi'e  of  the  details  of  the  petrogra]:)hy,  he 
is  referred  to  the  description  of  the  Archean  rocks  in  Monographs  XXXVI 
and  XLV. 

DOLERITES  AND   METADOLERITES. 

The  dolerites  are  best  developed  in  the  gi-eat  area  of  Archean  mapped 
as  extending  from  Virginia  eastward  to  beyond  Biwabik,  although  occur- 
ring also  in  the  other  Archean  areas.  On  the  weathered  surface  they  show 
varied  shades  of  green  and  brown,  these  colors  grading  into  dirt}'  white  or 
light  yellow.  On  fresh  fracture  the  color  is  characteristiciall}'  some  shade 
of  green,  commonl}^  a  rich  dark  green.  The  texture  is  typically  ophitic, 
and  varies  from  coarse  to  fine.  Occasionall)''  a  luster  mottling-  or  poikilitic 
texture  is  present.  Under  the  microscope  the  jDlagioclase  feldspar  is 
obscured  by  alteration  products  consisting  largely  of  epidote,  mica,  quartz, 
and  kaolin.  The  feldspar  laths  interlock  to  give  the  ophitic  arrangement. 
The  interstices  are  occupied  b)^  secondary  hornblende,  fine-grained  feld- 
spar, and  their  alteration  products,  mica,  chlorite,  and  zoisite.  In  addition 
there  are  present  minute  quantities  of  ilmenite,  sphene,  and  magnetite. 
The  rock  was  originally  a  typical  dolerite,  but  the  alterations  make  the 
term  metadolerite,  or  altered  dolerite,  appropriate  for  the  greater  mass  of  it. 

BASALTS    AND    METABASALTS. 

The  basalts  are  most  closely  associated  witli  the  dolerites — in  fact, 
grade  into  them — and,  like  them,  occur  in  the  greatest  quantity  in  the 
great  eastern  area  of  the  Archean.  The  conspicuous  features  by  which  the 
basalts  are  distinguished  from  the  dolerites  in  the  field  are  tlieir  fine  apha- 
nitic    and    porphj'ritic    textures.     Their    coloi-    on    fresh    fracture   also   is 


THE  BASEMENT  COMPLEX,  OR  ARCHEAN.  65 

frequently  a  somewhat  lighter  green  than  that  possessed  by  the  dolerites. 
Microscopically  the  basalts  show  much  altered  plagioclase-feldspar  pheno- 
crysts  and  occasional  quartz  phenocrysts  in  a  very  fine-grained,  although 
holocrystalline,  grouudmass.  The  alterations  of  the  feldspar  are  the  usual 
ones  to  sericite,  epidote-zoisite,  and  kaolin.  The  groundmass  consists 
mainly  of  feldspar  deeply  discolored  by  an  abundance  of  secondarv 
minerals,  including  chlorite,  zoisite,  iron  oxide,  and  i>,alcite.  No  original 
augite  is  present  and  little  or  no  secondary  hornblende.  The  texture  is 
sometimes  of  the  fine,  even  grade  known  as  "cryptocrystalline"  or  "micro- 
crystalline,"  and  at  others  the  irregular  mottled  kind  known  as  "micro- 
poikilitic."  With  these  textures  may  occasionally  be  seen  a  slight  arrange- 
ment of  the  finer  constituents,  particularly  the  iron  oxide  or  plag'ioclase 
laths  (pilotaxitic  texture),  in  such  a  manner  as  to  suggest  flowage  lines  of 
a  lava.  Rai'ely,  also,  the  groundmass  shows  a  spherulitic  texture  (625 
paces  north  of  the  southeast  corner  of  sec.  36,  T.  59  N.,  R.  16  W.),  and, in 
this  case  there  seeins  to  be  a  considerable  amount  of  quartz  in  phenocrysts 
and  in  the  groundmass.  The  amygdaloidal  texture  is  less  rare.  It  is 
best  seen  northeast  of  Biwabik.  The  amygdules  are  filled  with  quartz 
and  with  finely  fibrous  minerals  which  are  probably  zeolites,  the  latter 
not  infrequently  so  altered  as  to  be  visible  only  under  crossed  nicols. 
Other  common  structures  are  the  tuflfaceous  and  ellipsoidal  structures,  which 
appear  to  best  advantage  on  the  weathered  surface.  The  ellipsoidal  struc- 
ture is  typically  developed  north  of  Sparta.  The  ellipsoids  themselves 
consist  of  basalt,  and  vary  in  diameter  from  a  few  inches  to  one  or  two  feet. 
They  are  separated  by  narrow  bands  of  somewhat  lighter  or  darker  basalt. 
The  ellipsoidal  structure  is  one  supposed  to  have  been  induced  in  the  rock 
when  it  first  cooled  from  an  extensive  magma,  perhaps  subaqueous.  A 
similar  structure  has  been  observed  at  many  places  in  the  basic  igneous 
rocks  of  the  Lake  Superior  country  and  has  been  fully  described  by 
Clements  for  the  Crystal  Falls  and  Vermilion  districts."  The  figures  in 
the  Vermilion  monograph  representing  the  structure  in  the  Vermilion  dis- 
trict would  represent  the  structure  equall}^  well  for  the  Mesabi  district. 

As  in  the  case  of  the  diabase,  the  alteration  of  the  basalts  has  been 
of  such  a  nature  as  to  make  the  name  "metabasalt"  appropriate  for  most  of 
the  phases. 

«Mon.  V.  S.  Geol.  Survey  Vols.  XXXVI  and  XLV. 
MON  XLIII — 03 5 


66  THE  MESABI  IRON-BEARING  DISTRICT. 


DIORITES. 


The  diorites  are  dark-gray  or  green  rocks,  which  on  weathered  surfaces 
resemble  hornblende-granite.  Their  principal  constituents  are  plag-ioclase 
feldspar  and  hornblende,  but  there  are  alteration  products,  including  mica, 
chlorite,  epidote-zoisite,  quartz,  kaolin,  etc.  The  texture  varies  fi-oin  coarse 
to  fine  and  from  granitic  to  porphyritic.  In  the  latter  case  the  hornblendes, 
frequently  showing  enlargement,  are  the  porphyi'itic  constituents. 

By  diminution  in  the  amount  of  feldspar  and  increase  in  the  amount 
and  coarseness  of  the  hornblende  present  the  diorites  grade  into  coarse 
hornblende  rocks,  which  consist  almost  entirely  of  coarse,  stumpy,  dark- 
green  hornblende  crystals  with  random  arrang-ement.  The  interstitial 
material  is  plagioclase  feldspar  and  is  exceedingly  sparse.  Ilmenite, 
showing  alteration  to  deeply  colored  sphene,  is  present  both  in  the  horn- 
blende and  in  the  matrix.  The  hornblende  rocks  diifer  from  ihe  diorites 
onlv  in  the  relatively  greater  amount  of  hornblende  present,  and  really 
constitute  but  a  special  phase  of  the  diorites.  They  might  perhaps  be 
called  "hornblendites,"  but  these  rocks  usually  contain  a  small  amount  of 
augite.  Tliey  would  fall  under  the  general  group  of  "perknites,"  a  name 
recently  suggested  by  Turner"  for  rocks  consisting  largely  or  entirely  of 
monoclinic  amphibole  or  pyroxene,  or  l^oth. 


PERIDOTITE. 


Peridotite  has  been  found  in  exploration  work  in  sec.  33,  T.  59  N.,  R. 
15  W.  It  is  not  certain  that  the  rock  found  in  this  exploration  is  in  place 
and  not  a  float  from  the  north;  hence  it  is  but  mentioned. 

HORNBLENDIC      SCHISTS. 

The  hornblendic  schists  are  best  developed  in  the  area  north  of 
Mountain  Iron,  but  they  are  found  throughout  the  areas  mapped  as  Archean. 
In  typical  form  they  are  rich  dark-green  rocks,  sometimes  almost  black, 
and  show  many  brilliant  reflections  of  hornblende  cleavage  faces.  Many 
variations  of  the  type  are  to  be  seen.  The  schist  may  have  a  rather  light 
gi'ayish-green  color,  or  it  may  take  on  a  yellowish  color,  due  to  the  presence 
of  a  considerable  amount  of  feldspar  in  the  rock.  Tlie  texture  varies  from 
coarse  to  flue.     Tlie  hornblende  crystals  have  a  tendency  to  lie  with  their 

"Jour.  (;eol.,  Vdl.  IX,  l!)i)l.  ]<\k  507-.M1. 


T.  58  N. 


■N  85  -1 


THE  BASEMENT  COMPLEX,  OR  ARCHEAN.  67 

columnar  directions  almost  parallel,  giving  tlie  rock  its  schistosity  or 
cleavage.  When  broken  with  a  hammer  the  parting  of  the  rock  parallel 
to  the  schistosity  is  observed  not  to  follow  a  plane,  but  to  be  everywhere 
parallel  to  the  columnar  crystals.  The  pieces  of  the  rock  broken  off 
roughly  resemble  in  shape  and  dimensions  the  individual  hornblende 
crystals  making-  up  the  rock.  Each  of  the  pieces  of  rock  broken  off 
exhibits  the  same  glistening  faces  of  hornblende,  showing  that  in  the 
breaking  the  elongated  crystals  have  parted  along  their  mineral  cleavage 
planes. 

Under  the  microscope  the  hornblende  appears  in  fresh  green  columnar 
crystals,  almost  certainly  secondary,  with  a  tendency  to  parallelism  of  their 
long  axes,  although  many  crystals  are  not  so  arranged.  Rarely  the  horn- 
blende appears  in  two  forms  in  the  same  slide — in  large  stumpy  crystals 
almost  as  wide  as  long  and  with  no  parallel  arrangement,  and  in  slender 
columnar  forms  with  parallel  arrangement.  An  interesting  feature  here  is 
that  the  parallel  columnar  hornblendes  result  occasionally  from  the  parting 
or  slicing  of  the  large  stumpy  hornblende  crystals  along  their  cleavage 
planes;  that  is,  under  pressure  the  large  hornblendes  have  parted  along 
their  cleavage  planes,  yielding  a  large  number  of  slices  which  have  much 
greater  length  than  breadth  or  thickness  and  have  been  arranged  parallel. 

The  hornblende  crystals  lie  in  a  fine-grained  and  much  discolored 
matrix  of  feldspar,  chlorite,  and  epidote-zoislte,  these  minerals  showing  a 
variety  of  proportions  in  different  rocks.  The  usual  accessory  minerals, 
including  magnetite  and  calcite,  are  to  be  observed. 

With  increase  in  the  amount  of  feldspar  the  hornblendic  schist  grades 
into  amphibolite.  With  increase  in  the  amount  of  quartz  the  hornblendic 
schists  may  become  almost  indistinguishable  from  the  hornblendic  g'ray- 
wackes  of  Lower  Huronian  age  which  are  found  associated  with  horn- 
blendic schists  north  of  Mountain  Iron  and  Hibbing  (see  pp.  69-70),  and, 
indeed,  it  is  not  unlikely  that  some  of  the  rocks  containing-  considerable 
quartz  and  feldspar,  here  described  as  hornblendic  schists,  may  themselves 
have  been  derived  from  the  alteration  of  sediments.  Most  of  the  horn- 
blendic schists,  however,  have  unquestionably  been  derived  from  the 
alteration  of  basic  igneous  rocks  of  the  Archean  described  above,  and 
usually,  moreover,  have  received  their  most  characteristic  feature  through 
the  metamorphic  effect  of  the  Lower  Huronian  granite.     In  the  Mesabi 


68  THE  MESABI  IRON-BEARING  DISTRICT. 

distiict  the  actual  transition  from  the  massive  basic  rocks  to  the  horu- 
blendic  schists  may  be  observed  in  but  few  places,  but  along  the  contact 
between  the  gi-anite  and  the  basic  igneous  rocks  of  the  Archean  hom- 
blendic  schists  are  everywhere  abundant,  and  in  the  Vermilion  district  to 
the  north  hornblendic  schists  of  the  same  character,  age,  and  associations 
may  be  observed  in  all  stages  of  alteration  from  basic  igneous  rocks, 
brought  about  mainly  and  finally  through  the  intrusion  of  granite. 

MICACEOUS  SCHISTS   AND   CHLORITIC  SCHISTS. 

These  rocks  may  be  seen  throughout  the  Archean,  but  to  special 
advantage  in  the  great  Archean  area  eastward  from  Virginia  and  north  of 
Biwabik.  When  typically  developed  they  consist  largely  of  chlorite  and 
mica,  principally  biotite  but  partly  muscovite,  with  locally  more  or  less 
talc,  l}"iug  in  a  groundmass  composed  mainly  of  feldspar  with  subordinate 
amounts  of  quartz,  hornblende,  magnetite,  ilmenite,  and  zoisite. 

A  small  part  of  the  micaceous  schists  has  resulted  from  the  alteration 
of  the  acid  igneous  rocks  of  the  Archean,  but  the  greater  part  of  the  mica- 
ceous schists  and  the  chloi-itic  schists,  like  the  hornblendic  schists,  has 
developed  from  the  basic  igneous  rocks  of  the  Archean.  Why  hornblendic 
schists  should  develop  in  some  places  and  chloritic  and  micaceous  schists  in 
others  is  not  always  known,  but  in  general  it  seems  to  be  true  that  the 
hornblendic  schists  are  characteristic  of  the  contact  of  the  granite  with  the 
basic  igneous  rocks  of  the  Archean,  while  the  chloritic  and  micaceous  schists 
are  characteristic  developments  from  the  folding  and  mashing  of  the 
Archean  rocks  away  from  the  granite. 

GRANITE  AND   PORPHYRITIC  RHYOLITE. 

The  Archean  acid  igneous  rocks  include  porphyritic  rhyolite,  porphy- 
ritic  granite,  and  granite.  The  two  former  may  be  conveniently  referred 
to  as  "porphyries."  Between  Virginia,  Sparta,  and  Eveleth  are  three  small 
areas  of  Archean  porphyries.  The  one  mapped  as  lying  mainly  in  sec.  22, 
T.  58  N.,  R.  17  W.,  is  a  porphyritic  granite.  The  rock  weathers  white, 
light  green,  and  dirty  yellow.  The  phenocrysts  of  quartz  and  plagioclase 
feldspar  stand  in  a  fairly  fine-grained  groundmass  of  quartz  and  feldspar  in 
which  appears  a  considerable  quantity  of  secondary  sericite,  chlorite,  and 
quartz,  indicating  considerable  alteration.  The  large,  clear  phenocrysts  of 
quartz  stand  out  like  eyes.     An  almost  identical  rock  has  been  found  in  the 


THE  BASEMENT  COMPLEX,  OR  ARCHEAM.  69 

Vermilion  iron  district,  and  in  both  the  Vermihon  and  Mesabi  districts  the 
rock  has  been  designated  in  the  field  the  "white-eyed  porphyry."  The 
mashing  to  which  the  rock  has  been  subjected,  together  with  the  develop- 
ment of  secondary  mica  and  chlorite,  has  in  places  altered  the  porphyries  to 
chloritic  schists  and  micaceous  schists,  either  kind  being  more  or  less  talcose. 

The  porphyry  mapped  in  sees.  16  and  21,  T.  58  N.,  R  17  W.,  and 
along  the  quarter  line  of  sec.  29,  T.  68  N.,  R.  17  W.,  is  almost  the  same  in 
texture  and  mineral  content  as  the  one  just  described,  except  that  the  phe- 
nocrysts,  instead  of  being  both  quartz  and  feldspar,  are  plagioclase  feldspar 
alone.  Moreover,  a  considerable  amount  of  secondary  calcite  and  pyrite 
are  to  be  observed  in  the  groundmass.  The  rock  is  called  a  feldspar- 
porphyry.  Like  the  porphyr}^  above  described,  it  shows  much  mashing, 
and  by  its  alteration  has  yielded  chloritic,  muscovitic,  and  talcose  schists. 

Near  the  west  line  of  sec.  25,  T.  59  N.,  E,.  18  W.,  is  a  dense  dark-gray 
porphyry  associated  with  hornblendic  schist.  Under  the  microscope  the 
acid  feldspar  phenocrysts  show  zonal  cloudy  alterations  to  muscovite  and 
kaolin.  The  feldspars  lie  in  a  fine  but  imeven  grained  matrix  of  feldspar 
and  quartz,  with  a  subordinate  amount  of  muscovite,  chlorite,  kaolin,  and 
zoisite. 

Seventy  paces  north  of  the  southeast  corner  of  sec.  6,  T.  58  N.,  R.  16  W., 
there  is  an  exposure  of  biotite-granite.  The  surrounding  rock  is  Lower 
Huronian  slate  and  graywacke,  and  we  have  no  evidence  that  the  granite 
itself  is  Archean.  However,  it  is  a  considerable  distance  from  the  Lower 
Huronian  granite,  and,  on  the  other  hand,  not  far  from  the  granites  and 
porphyries  whose  age  is  known  to  be  Archean,  and  it  has  been  mapped  as 
Archean.  Orthoclase  feldspar  and  rather  abundant  quartz  form  the  mass 
of  the  rock.  With  this  is  a  liberal  sprinkling  of  biotite  and  less  greenish 
hornblende.     The  texture  is  typically  granitic,  although  rather  fine. 

SEDIMENTARY    ROCKS. 

North  of  jVIountain  Iron  and  Hibbing,  and  westward,  frag'mental  rocks 
are  intricately  mingled  with  the  Ai'chean  igneous  rocks  (see  Pis.  IV  and  V). 
There  is  no  positive  evidence  to  show  whether  these  rocks  are  Upper 
Huronian,  Lower  Huronian,  or  Archean.  As  they  are  closely  folded  with 
the  Archean,  it  is  probable  that  they  are  not  Upper  Huronian.  Litholog- 
ically  they   resemble  the   Lower  Huronian    rocks,    and    hence    they   are 


70  THE  MESABI  IRON-BEARING  DISTRICT. 

described  in  couuection  with  the  Lower  Huronian.  Nowhere  in  the 
district  have  sediments  been  found  which  are  demonstrably  of  Archean  age. 
However,  certain  facts  seem  to  show  that  sedimentary  rocks  of  Archeau 
age  are  actually  present  in  the  district.  In  the  basal  conglomerate  of  the 
Lower  Huronian  were  found  a  few  somewhat  doubtful  slate  fragments  and 
a  single  pebble  of  what  is  taken  to  be  a  fine-grained  grit  containing  grains 
of  quartz,  feldspar,  and  iron  oxide.  While  careful  search  has  failed  to 
reveal  the  counterparts  of  these  rocks  in  the  true  Arcliean,  it  is  possible 
that  in  the  future  they  will  be  found.  Indeed,  it  is  not  impossible  that 
certain  of  the  altered  sediments  included  in  the  Archean  and  mapped  as 
Lower  Huronian  may  be  truly  of  Archean  age.  On  the  other  hand,  the 
sedimentary  fragments  in  the  conglomerate  of  the  Lower  Huronian  may 
have  been  brought  in  from  distant  areas  and  the  Archean  of  the  Mesabi 
di.strict  in  itself  lack  them.  It  makes  little  difference  which  is  the  case,  for 
it  is  known  that  beneath  the  Lower  Huronian  rocks  in  the  Lake  Superior 
region  are  other  subordinate  sedimentary  rocks  associated  with  what  has 
been  mapped  in  the  past  as  Archean.  The  pebble  in  the  conglomerate 
here  described  offers  additional  evidence  of  this  fact.  Whether  or  not 
small  areas  of  these  Archean  sedimentary  rocks  be  found  in  place  in  the 
narrow  confines  of  the  Mesabi  district  is  a  matter  of  small  importance. 

STRUCTUEE. 

The  Archean  rocks  of  the  district,  being  igneous  throughout,  have  only 
such  structures  as  are  characteristic  of  massive  and  schistose  igneous  rocks 
The  original  igneous  structures  have  been  mentioned  above.  While  most 
of  the  Archean  rocks  show  some  cleavage,  perhaps  about  half  have 
enough  cleavage  to  warrant  calling-  them  schists.  In  general  the  plane 
of  cleavage  is  nearly  vertical  and  strikes  parallel  to  the  range,  aliout  N. 
60°  E.  The  hornblendic  schists  north  of  Mountain  Iron  have  a  cleavag-e 
of  a  linear  parallel  type,  and  the  lines  of  the  cleavage  dip  steeply  to  the 
northeast.  In  addition  to  cleavage  there  are  many  joints  and  faults  with 
dis))lacoments  of  a  few  inches  or  feet,  but  no  regular  systems  have  been 
determined. 

RELATIONS  TO  OTHER  SERIES. 

The  Archean  rocks,  both  basic  and  acid,  form  a  basement  u])on  wjiich 
the  sedimentary  rocks  of  the  region  wei'e  deposited,  and  hence  between  the 
Arclicaii  and  tlir  nvo'K'iiiL;'  rocks  is  a  stnictural  luiconfonuitv. 


IK' 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH  XLIII  PL.V. 


LiUS  BIEM  aCO.LlTH.K."' 


THE  BASEMENT  COMPLEX,  OR  ARCHEAN.         71 

The  sediraentary  rocks  now  lying  next  to  the  Archean  are  Lower 
Huronian  for  a  part  of  the  district  and  Upper  Hiironian  for  another  part. 
This  results  from  the  fact  that  the  Upper  Huronian  is  unconformably  above 
the  Lower  Huronian  and  laps  over  the  Lower  Huronian  onto  the  Archean. 
The  Lower  Huronian,  near  its  contact  with  the  Archean,  is  a  coarse 
conglomerate,  containing  large  pebbles  and  bowlders  of  ^he  kinds  of  rocks 
found  in  the  Archean,  with  the  exception  of  some  of  the  schists,  which  were 
formed  by  the  mashing  of  Archean  rocks  subsequent  to  the  deposition  of 
the  conglomerate.  The  actual  contact  of  the  Upper  Huronian  and  Archean 
is  drift  covered,  but  from  the  known  fact  that  the  Upper  Huronian  is 
unconformably  above  the  Lower  Huronian  and  the  Lower  Huronian  is 
unconformably  above  the  Archean,  it  is  certain  that  the  Upper  Huronian 
rests  unconformably  iipon  the  Archean. 

The  Archean  along  its  entire  northeastern  edge  is  in  contact  with 
granite  which  is  intrusive  into  the  Archean  rocks.  Actual  contacts  of  the 
two  are  to  be  observed  in  a  number  of  places,  and  at  such  places  the  Archean 
greenstones  become  micaceous  or  hornblendic. 

While  some  of  the  schists  are  clearly  the  altered  equivalents  of  the 
Archean  igneous  rocks  which  have  yielded  pebbles  to  the  conglomerates  at 
the  base  of  the  Lower  Huronian,  others  of  the  schists  have  not  been  proved 
to  have  resulted  from  the  alteration  of  Archean  rocks,  but  are  supposed  to 
be  Archean  from  their  lithological  similarity  to  such  rocks.  In  spite  of 
such  similarity,  some  of  the  schists  may  be  of  Lower  Huronian  age.  To 
this  doubtful  group  belong  a  part  of  those  north  of  Mountain  Iron,  north- 
west of  Hibbing,  and  in  Rs.  22  and  23  W. 


CHAPTER  IV. 

THE  LOWER  HURONIAN  SERIES. 

DISTRIBUTION. 

Sedimentary  rocks  of  Lower  Huronian  age  appear  in  two  considerable 
areas  in  the  Mesabi  district.  One  witli  an  average  width  of  perhaps  a  niile 
extends  from  Eveleth  northeast  to  Biwabik ;  the  other,  somewhat  less  than 
a  mile  in  width,  extends  from  near  the  Duluth  and  Iron  Range  Railroad 
northeast  to  near  the  center  of  sec.  11,  T.  59  N.,  R.  14  W.  In  the  former 
belt  there  are  areas  of  green  schist  forming  the  cores  of  the  hills.  One  of 
them  has  been  mapped,  but  others,  while  their  presence  is  known  by 
isolated  exposm'es,  are  not  sufficiently  exposed  to  warrant  their  separation 
on  the  map.  A  number  of  small  patches  of  Lower  Huronian  sediments 
are  known  also  in  other  parts  of  the  district,  as  follows:  East  of  Biwabik, 
in  the  northern  portion  of  sec.  1,  T.  58  N.,  R.  16  W.;  north  of  Biwabik,  in 
sec.  34,  T.  59  N.,  R  16  W. ;  bordering  the  Archean  north  of  the  Genoa  mine 
at  Sparta;  northwest  of  Virginia,  along  the  line  between  sees.  31  and  32,  T. 
59  N.,  R.  17  W. ;  northeast  of  Virginia,  near  the  east  side  of  sec.  34,  T.  59 
N.,  R.  17  W. ;  bounding  the  Archean  north  of  Mountain  Iron,  in  sec.  34, 
T.  59  N.,  R.  18  W. ;  intricately  mixed  with  hornblendic  schists  and  acid 
intrusives  in  a  belt  running  through  sees.  28,  27,  22,  and  23,  T.  59  N.,  R. 
18  W.  (see  PI.  V);  northwest  of  Hibbing,  in  a  narrow  belt  bounding  the 
Archean  in  sees.  26,  34,  and  35,  T.  58  N.,  R.  21  W.,  also  in  deep  drill  hole 
beneath  quartzite  1,025  paces  north,  665  paces  west,  sec.  35,  T.  58  N.,  R. 

21  W.;  in  the  area  mapped  as  Archean  in  sees.  19,  30,  and  20,  T.  57  N.,  R. 

22  W.;  and  near  the  contact  of  the  hornblendic  schist  with  the  granite  near 
the  north  line  of  sec.  2,  T.  56  N.,  R.  23  W. 

Granite  of  Lower  Huronian  age  forms  the  core  of  the  Giants  range 
and  is  exposed  on  its  upper  slopes  from  Grand  Rapids  eastward  to  near  the 
east  line  of  R.  14  W.,  with  only  one  break,  nortli  of  Mountain  Iron,  where 
it  is  interrupted  for  a  short  distance  by  Arcliean  hornblendic  schists.  The 
granite  tlms  bounds  on  the  north  the  other  formations  for  most  of  the  district. 
Our  detailed  work  has  not  gone  farther  north  than  the  granite  boundary. 

72 


THE  LOWER  HURONIAN  SERIES. 


73 


A  dike  of  Lower  Huronian  porphyiy  appears  northwest  of  Biwabik, 
in  the  northern  part  of  sec.  3,  T.  58  N.,  R.  16  W.     The  porphyry  in  sec.  25, 


R.17W. 


R.i7W. 


Scale 


LEGEND 

ALGONKIAN 

UPPER  HURONIAN 


Ab 


Biwabik  (iron- 
bearing)  formatioa 


Apq 


Pokegama 

quartzite 

LOWER  HURONIAN 


Ahg 


Graywacke  slate 
and  congrlomerate 


ARCHEAN 


Basalt  and 
porphyry 


m 

Outcrop 

^ 

Outcrop 
(witJi  bedding) 

» 

Out<;rop 
(amglomerote) 

a 

0  K  ImUe 

Contour  interval  20  fee'^ 

Fig.  1.— Detail  map  showing  distribution  of  upper  Huronian,  Lower  Huronian,  and  Archean  rocks  northeast  of  Eveleth. 

T.  59  N.,  R.  18  W.,  described  on  page  69  with  the  Archean,  may  be  Lower 
Huronian,  but  there  is  no  evidence  one  way  or  the  other. 


74  THE  MESABI  IKON-BEARING  DISTRICT. 

KIXDS  OF  ROCKS. 

The  Lower  Huronian  rocks  are  both  sediinentarj'  and  igneous.  The 
sedimentary  rocks  include  interbedded  slate,  graywacke,  and  conglomerate, 
and  the  igneous  rocks  include  granite  and  porphyry. 

GRAYWACKES  AND   SLATES. 

The  following  description  applies  to  the  normal  phase  of  graywacke 
and  slate  making  up  the  bulk  of  the  sedimentary  portion  of  the  series.  The 
highly  metamorphosed  phases  caused  by  the  metamorphism  of  the  granite 
are  described  in  a  separate  section. 

The  interbedded  gray  wackes  and  slates  form  the  great  bulk  of  the  sed- 
iments. They  are  dull,  dark-gray  and  dark-green  rocks  which  usually 
weather  to  a  somewhat  lighter  green  or  gray  or  to  a  dirty  light  yellow. 
The  grain  is  usually  fine,  although  it  varies  considerably.  The  bedding, 
shown  by  both  color  and  texture,  is  conspicuous.  Parallel  to  the  bedding 
a  secondary  cleavage  has  been  developed.  As  a  result  of  variation  in  tex- 
ture, bedding,  and  secondary  cleavage,  there  appear  all  gradations  between 
metamorphosed  coarse  graywackes,  banded  graywackes,  and  finely  fissile 
slates.  Alf)ng  the  parting  plane  of  some  of  the  graywackes  and  slates  may 
be  seen  glistening  plates  of  mica  or  chlorite,  conspicuous  because  of  the  fact 
that  they  appear  in  separate  spangles  on  the  dark  background  rather  than 
in  continuous  layers,  although,  indeed,  some  of  the  more  fissile  slates  show 
mica  and  chloi'ite  in  the  continuous  layers  characteristic  of  slates. 

Under  the  microscope  the  graywackes  and  slates  show  little  uniformity 
in  texture  and  mineralogical  composition.  A  composite  slide  from  the  less 
altered  graywackes  would  show  angular  to  subaugular  grains  of  quartz  and 
feldspar  in  about  equal  quantity  and  of  rather  unifoi-m,  small  size,  cemented 
by  a  sparse,  ill-defined  matrix  of  the  same  material,  in  which  there  is  nuich 
chlorite  and  micaceous  material  and  cloudy  alteration  products  of  the  feld- 
spar. While  the  particles  are  not  well  rounded,  their  general  aspect  leaves 
no  doubt  as  to  their  clastic  character.  Certain  slides  show  a  predominance 
of  quartz  grains  and  others  a  predominance  of  feldspar  grains.  Certain 
slides  have  almost  no  cementing  material ;  in  others  it  is  so  abundant  as  to 
make  the  clastic  grains  look  almost  like  phenocrysts.  In  certain  slides, 
again,  tlio   matrix   is   almost   entirely    an   ill-defined   greenish   I'hloritic   or 


THE  LOWER  HURONIAN  SERIES.  75 

micaceous  siibstance ;  in  others,  a  fiiie-grained  cloudy  alteration  of  feldspar 
witli  little  of  this  material.  The  chlorite  and  mica  in  the  matrix  are  in 
large,  distinct  plates  parallel  to  the  bedding.  These  are  the  ones  which 
appear  so  conspicuously  on  the  parting  planes  of  the  graywackes  above 
referred  to. 

The  slates  under  the  microscope  show  an  exceedingly  fine  felty  mass 
of  quartz  and  feldspar  almost  obscured  by  an  aggregate  of  micaceous  and 
chloritic  substances.  In  other  words,  they  show  the  ordinary  features  of 
typical  slates.  A  great  variety  of  rocks  intermediate  between  the  gray- 
wackes and  slates  show  microscopical  features  intermediate  between  those 
above  described. 

In  certain  areas  iron  pyrites  is  fairly  abundant  in  both  the  slates  and 
the  graywackes.  This,  while  occasionally  fresh,  is  for  the  most  part  altered 
to  iron  oxide,  which  retains  the  cubic  form  of  the  pyrites,  or,  if  altered  to 
iron  ore,  is  weathered  out  altogether,  being  represented  only  by  iron-stained 
cavities  which  frequently  retain  the  cubic  form.  Iron  pyrites  may  be 
especially  well  observed  in  the  SW.  |  sec.  22,  T.  58  N.,  R.  17  W. 

The  graywackes  and  slates  abovp  described  have  resulted  from  the 
alteration  of  fine  mud  and  feldspathic  sand  deposits.  The  metamorphism 
has  consisted  in  their  cementation  into  hard  rocks,  which  has  been  brought 
about  by  the  recrystallization  of  the  finer  materials  in  the  background  and 
perhaps  the  infiltration  of  quartz  from  without,  and  by  the  abundant 
development  of  chloritic  and  micaceous  materials.  Some  of  the  mica, 
especially  that  in  separate  clear-cut  plates,  may  have  been  originally 
deposited  in  its  present  position,  but  most  of  it,  and  especially  that  in 
continuous  sheets  on  the  parting  surfaces,  is  undoubtedly  a  secondary 
development  due  to  dynamic  movement  in  the  rock.  In  general,  the 
mashing  of  the  rock  has  not  been  sufficient  to  develop  any  secondary 
structure  inclined  to  bedding,  and  its  main  effect  has  been  in  developing 
micaceous  minerals  parallel  to  bedding. 

CONGLOMERATES. 

The  conglomerates  are  perhaps  the  most  interesting  of  the  Lower 
Huronian  sediments.  They  are  most  abundantly  and  typically  exposed 
in  a  belt  running  from  the  cut  along  the  Duluth  and  Iron  Range  Rail- 
road in  sec.  22,  T.  58  N.,  R.  17  W.,  southwest  through  sees.  22  and  21 


76  THE  MESABI  IRON-BEARING  DISTRICT. 

into  sees  20  and  29,  T.  58  N.,  R.  17  W.  Similar  conglomerates  are  known 
in  small  patches  bordering  the  greenstones  north  of  the  Grenoa  mine,  at 
Sparta. 

The  conglomerates  are  massive  rocks  for  the  most  part,  with  various 
shades  of  green  on  fresh  surface  and  a  lig'hter  green  on  the  weathered 
surface.  The  pebbles  vary  in  diameter  from  6  inches  to  a  small  fraction 
of  an  inch,  in  kind  they  are,  for  the  most  part,  identical,  both  macro- 
scopically  and  microscopically,  with  the  rocks  in  the  Archean  above 
described,  including  diabases,  basalts,  and  granite-porphyries.  The  more 
basic  pebbles  are  in  greater  quantity  than  the  acid  ones.  One  of  the 
most  characteristic  pebbles  is  a  peculiar,  purplish,  dark-green  porphyritic 
basalt  in  which  the  phenocrysts,  originally  of  feldspar,  are  now  spots  of 
greenish  chloritic  material.  The  conglomerates  have  a  fine-grained  green 
matrix,  which  was  probably  originally  lai'gely  of  feldspar  and  quartz,  but 
which  is  now  almost  obscured  by  chlorite  and  sericite  alterations.  The 
common  green  fragments  and  the  green  matrix  of  the  conglomerates  make 
the  name  "greenstone-conglomerate"  very  appropriate,  and  this,  indeed,  is 
what  the  rock  has  been  called  during  the  field  work.  In  walking  through 
the  railroad  cut  above  referred  to,  unless  one  looks  very  closely  he  is 
likely  to  suppose  the  rock  to  be  an  original  basic  igneous  one.  In  other 
places  many  of  the  pebbles  of  the  basic  igneous  rocks  weather  to  a  salmon 
pink,  giving  the  impression  that  the  rock  is  made  up  largely  of  porphyry 
pebbles.  An  examination  shows  that  the  apparently  acid  fragments  are 
really  basic,  while  the  true  porphyries  weather  grayish  green  and  look 
basic. 

In  addition  to  the  common  pebbles  above  named,  there  ^jpear  a  few 
pebbles  of  white  and  greenish-gray  chert,  which  may  represent  altered 
slate  (45494).  Close  examination  of  these  fails  to  determine  whether  or 
not  they  are  sedimentary  slates,  but  one  or  two  fragments  are  seen  to  have 
a  very  fine  banding,  which  may  indicate  sedimentary  origin.  In  the  Duluth 
and  Iron  Range  Railroad  cut,  also,  one  pebble  was  found  which  may  be  a 
fine  gi'it  or  graywacke.  It  is  a  greenish-gray,  fine-grained  rock  made  up 
of  roundish  and  subangular  grains  of  quartz  and  much-altered  feldspar  in 
an  abundant  fine-grained  matrix  of  similar  materials,  obscured  by  greenish 
alteration  products.  Throughout  the  rock  are  little  specks  of  iron  oxide. 
One  of  these  appears  on  the  weathered    sm-face  like   a  little  fragmental 


THE  LOWER  HURONIAN  SERIES.  77 

grain  of  jasper,  and  was,  indeed,  the  feature  which  first  attracted  attention 
to  the  pebble. 

The  presence  of  this  possible  sediment  in  pebbles  in  the  conglomerate 
at  the  base  of  the  Lower  Huronian  series  may  indicate  the  presence  of  still 
older  sedimentar}^  rocks  somewhere  in  this  area.  Such  an  older  sedimen- 
tary formation  has  been  found  in  other  districts  of  the  Lake  Superior 
region,  the  Vermilion  and  Marquette,  but,  with  the  possible  exception  of 
certain  doubtful  sediments  north  of  Mountain  Iron,  no  such  rocks  have 
been  found  in  the  Archean  within  the  limits  of  this  district.  Further 
search  may  reveal  them  in  small  patches,  but  certainlj^  tliey  occupy  no 
considerable  areas. 

In  the  NW.  i  sec.  34,  T.  58  N.,  R.  17  W.,  just  north  of  the  Genoa 
mine,  patches  of  conglomerate  may  be  observed  in  the  southerly  exposures 
of  the  massive  Archean  greenstones.  On  weathered  surface  the  light-gray, 
green,  or  pink  angular  to  subangular  fragments  stand  out  consjDicuously 
from  a  dark-green  matrix.  On  fresh  fracture  fragments  and  matrix  have  a 
dark-green  color  and  can  not  be  separated.  They  both  resemble  the 
underlying  Archean  basalt.  The  conglomerate  is  associated  with  a  small 
quantity  of  banded  rock  of  the  same  general  character,  which  is  probably 
an  altered  graywacke  associated  with  the  conglomerate.  The  rock  next  to 
the  south  is  Pokegama  quartzite,  but  the  conglomerate  has  not  been 
actually  connected  with  the  Pokegama  quartzite,  and  because  of  its  meta- 
morphosed character  and  similarity  to  the  Lower  Huronian  conglomerates 
of  other  areas  it  is  here  described. 

The  conglomerates,  in  common  with  the  rest  of  the  Lower  Huronian 
rocks,  have  suffered  metamorphism,  but  the  extent  of  the  alteration  varies 
greatly  from  place  to  place.  East  of  Mariska,  in  the  railway  cut  referred 
to,  the  rocks  show  only  re  crystallization  of  the  mineral  particles,  without 
marked  development  of  schistosity.  The  alteration  of  the  minerals  is  the 
same  as  that  described  above  for  the  various  rocks  of  the  Archean.  To 
the  southwest  of  this  cut  the  conglomerates  have  been  much  squeezed  and 
are  now  very  schistose.  The  recrystallization  accompanying  the  squeezing, 
has  made  the  rocks  very  chloritic  and  micaceous,  and,  in  many  cases  at  least, 
lias  completely  obliterated  the  clastic  texture  in  the  finer-grained  portions. 
The  pebbles  have  been  elongated  in  the  plane  of  schistosity  (vertical  and 
striking  N.  60°  E.),  and  on  the  weathered  surface  stand  out  in  lenticular  and 


78  THE  MESABI  IRON-BEARIiNG  DISTRICT. 

oval  forms  from  the  finer,  more  schistose,  and  more  easily  eroded  mati'ix. 
Rocks  of  this  character  may  be  traced  into  schistose  rocks  in  which,  in  peb- 
bles and  matrix  alike,  nearly  every  vestige  of  sedimentary  texture  has  been 
lost. 

GRANITES  AND   PORPHYRIES   (Porphyritic  Granites  and  Porphyritic  Rhyolites). 

Lower  Huronian  granites  form  a  continuous  belt  along  the  higher  parts 
of  the  Giants  range  from  near  the  east  line  of  R.  14  W.  to  the  west  end 
of  the  district,  except  for  a  short  distance  north  of  Mountain  Iron,  where 
they  are  cut  out  by  the  Archean  hornblende-schists.  They  also  make  up 
part  of  the  shores  of  Birch  Lake.  Over  this  great  area  the  granites  show 
considerable  lithological  complexity.  At  Birch  Lake  the  Lower  Huronian 
granites  are  coarse  gray  and  pink  hornblende-granites.  From  the  east  line 
of  R.  14  W.  to  the  neighborhood  of  Mountain  Iron  the  granites  are  similar 
to  those  on  Birch  Lake.  It  is  noticeable  that  the  coarser  phases  appear  in 
the  eastern  end  of  this  area.  The  hornblende  varies  in  abundance,  but 
is  usually  conspicuous.  Rarely,  as  near  the  Mailman  camps,  the  dark 
constituent  iis  augite  (45435)  instead  of  hornblende,  or  again  it  may  be 
partly  biotite.  The  feldspar  is  partly  orthoclase  with  Carlsbad  twinning, 
partly  microcline,  and  in  small  part  plagioclase,  and  all  of  it  shows  cloudy 
alteration  and  a  zonal  structure  indicating  two  stages  of  growth.  Occasion- 
ally also  it  appears  in  porphyritic  form.  The  hornblende  is  a  fresh  green 
variety.  Quartz  is  present,  but  very  sparsely;  indeed,  certain  phases  of 
the  rock  have  so  little  quartz  that  they  might  perhaps  be  called  syenites. 
A  characteristic  accessory  is  brown  sphene,  showing  in  places  stages  of  altera- 
tion from  ilmenite.  In  places  the  rock  becomes  very  slightly  gneissic,  and 
immediately  next  to  its  contact  with  the  Lower  Huronian  sediment  it 
becomes  very  fine  grained.  Next  to  the  contact  of  the  granite  with  the 
Keweenawan  gabbro  on  Birch  Lake  is  a  metamorphic  rock  resembling 
granite,  which  is  described  in  connection  with  the  gabbro. 

From  the  neigborhood  of  Mountain  Iron  westward  to  the  west  end  of 
the  district  the  preponderating  granite  is  somewhat  finer  grained  than  the 
granite  to  the  east,  possibly  somewhat  more  gneissic,  and  usually  of  a  pink 
color.  Certain  phases  of  this  finer  granite  are  similar  to  the  hornblende- 
granite  to  the  east,  but  by  far  the  larger  portion  shows  a  considerably 
greater  content  of    (juartz    and  a  smaller  content  of  the  basic   minerals. 


THE  LOWER  HURONIAN  SERIES.  79 

Instead  of  hornblendes  we  have  in  these  rocks  green  or  brown  biotite  and 
muscovite.  The  feldspar  is  partly  microline  and  partly  orthoclase,  as  in 
the  hornbleude-granites,  and  the  alteration  of  the  feldspar  is  about  the 
same.  As  in  the  hornblende-granites,  also,  the  feldspar  crystals  occasion- 
ally stand  out  in  porphyritic  fashion. 

Associated  with  these  two  prevailing  types  are  dikes  of  exceedingly 
fine-grained  pink  granite  showing  very  little  biotite.  They  may  be  well 
observed  in  the  cuts  along  the  main  line  of  the  Duluth  and  Iron  Range 
Railroad.  Other  dikes  are  pegmatitic  granite  consisting  of  a  pink  feldspar 
with  verjT-  abundant  quartz,  and  with  the  ferromagnesian  minerals  almost 
totally  lacking.  They  may  be  seen  to  advantage  at  the  upper  falls  of  the 
Prairie  River. 

In  connection  with  the  Lower  Huronian  granites  should  be  mentioned 
a  dike  of  feldspar-porphyry  intruding  the  Lower  Huronian  sediments  just 
northwest  of  Biwabik,  in  the  NW.  J  of  NW.  ^  sec.  3,  T.  58  N.,  R.  16  W. 
It  is  a  very  fine-grained,  grayish,  acid  rock  which  under  the  microscope 
shows  orthoclase-feldspar  phenocrysts,  now  much  altered,  lying  in  the 
usual  altered  matrix  of  quartz,  feldspar,  and  chloritic  material. 

North  of  Mountain  Iron,  in  sec.  34,  T.  59  N.,  R.  18  W.,  near  the 
contact  of  the  granite,  hornblende-schists,  and  Huronian  sediments,  is  an 
exposure  of  a  porphyritic  rh3^olite  in  which  quartz  and  feldspar  phenocrysts 
of  about  equal  abimdance  stand  in  a  fine-grained  but  holocrystalline  matrix 
of  quartz  and  feldspar  with  a  spherulitic  texture.  There  is  no  direct 
evidence  to  show  whether  the  rock  is  Lower  Huronian  or  Archean. 

At  1,400  steps  north  of  the  southeast  corner  of  sec.  16,  T.  59  N.,  R.  14 
W.,  in  the  Lower  Huronian  sediments,  is  a  dike,  about  25  paces  wide,  of  a 
dark-gray,  fine-grained,  schistose,  chloritic  and  hornblendic  granite.  The 
rock  under  the  microscope  is  seen  to  consist  of  orthoclase  feldspar  showing 
considerable  alteration  to  sericite,  kaolin,  and  zoisite,  which  is  in  about 
equal  abundance  with  hornblende  and  chlorite.  The  hornblende  is  a  green 
variety  showing  little  alteration  and  usually  having  crystal  foi-m.  The 
chlorite  is  secondary. 

In  sec.  11,  T.  59  N.,  R.  14  W.,  and  northwestward  for  a  mile  and 
perhaps  more  is  a  fine-grained  porphyritic  rhyolite  or  granite  which 
apparently  is  intnided  by  the  Lower  Huronian  granite.  As  to  its  age,  all 
that  can  be  said  is  that  it  is  older  than  the  Lower  Huronian  granite,  but 


80  THE  MESABl  IRON-BEARING  DISTRICT. 

whether  Archean  or  Lower  Hurouian  there  is  no  dh-ect  evidence.  Because 
of  its  close  association  with  the  granite  in  its  distribution  and  its  dissimihxrity 
to  the  Archean  porphyries,  it  is  described  in  this  connection  rather  than 
with  the  Archean.  The  distribution  of  this  rock  in  sec.  11  is  shown  by  the 
detailed  sketch  map  (PI.  VI),  The  rock  is  gray  or  pink,  fine  grained,  and 
contains  minute  red  and  gray  streaks.  Under  the  microscope  the  texture 
is  seen  to  varj^  from  porphyritic  to  granitic.  In  certain  slides  large  feldspar 
phenocrysts  stand  in  a  fine-grained  granular  matrix  of  quartz  with  a  sub- 
ordinate amount  of  feldspar.  The  feldspar  shows  in  places  strain  shadows, 
fracturing,  peripheral  granulation  or  even  total  granulation.  In  other 
slides  the  quartz  and  feldspar  particles  occur  in  about  equal  size.  It  is 
possible  that  the  porphyritic  texture  may  be  in  part  the  result  of  the 
granulation  of  a  part  of  the  constituents,  lea'sdng-  the  remainder  as  pheno- 
crysts in  a  granulated  background.  In  addition  to  the  quartz  and  feldspar 
there  are  present  greatl}^  varying  but  usually  small  quantities  of  horn- 
blende, biotite,  and  chlorite.  Under  the  microscope  the  similarity  of  this 
rock  to  the  altered  phases  of  the  Lower  Huronian  graywacke  is  striking. 
In  hand  specimens,  however,  they  may  be  discriminated. 

INCLUSIONS   IN    GRANITE. 

Through  a  considerable  portion  of  the  district,  and  particularly  north 
of  Mountain  Iron  and  Hibbing  and  from  there  westward,  there  are  found 
intricately  mixed  up  with  the  granite,  and  not  separable  on  a  map  of  ordi- 
nary scale,  small  quantities  of  hornblendic  schist,  chloritic  schist,  micaceous 
schist,  diorite,  basalt,  or  diabase,  or  their  metamorphosed  equivalents. 
Some  of  tlie  micaceous  schists  are  the  altered  equivalents  of  the  granite, 
and  some  of  the  diorites  are  also  apparently  genetically  connected  with 
the  granite;  such  rocks  are  of  Lower  Huronian  age.  Other  rocks,  par- 
ticularly the  hornblendic  schists,  chloritic  schists,  diabases,  and  diorites, 
are  intruded  by  the  granite,  and  these  might  be  either  Lower  Huronian  or 
Archean.  Still  others,  and  perhaps  the  larger  projiortion,  are  so  intricately 
mixed  with  the  granite  that  no  determination  of  their  age  can  be  made. 
Many  of  the  phases  can  be  duplicated  in  the  area  mapped  as  Archean,  and 
Indeed  the  line  between  the  granite  and  the  Archean  in  man}-  places  is 
determined  by  the  relative  abundance  of  these  rocks.  It  is  certain  therefore 
that  a  considerable  proportion  of  the  rocks  included  in  the  granite  are  of 
Archean  age. 


U.S.  GEOLOGICAL  SURVEY 


■MTONDGTiAPH  XLIII    PL.  VI 


LEGEND 
ALGONKIAN 

KEWEENAWAN 

|~AKgr     [ 

Granite 

UPPER  HURONIAN 


Cordierite  hornstone 
of  Virginia  formatioa 


Interstratified  slate 
of  Biwabik  (iron-bearing) 
formation 


Ferruginous  chert  of 

Biwabik  (iron-bearing) 

formation 


Apq 


Pokegama  quartzite 


LOWER  HURONIAN 


Ags 


Graywacke  and  slate 


Afp 


Feldspar  porphyry 


Outer  op 


X 


DETAIL  MAP  SHOAVIKG  DISTRIBUTION  OF  KEWEENAWAN, 

UPPER  HURONIAN  AND  LOWER  HURONIAN  ROCKS 

IN  THE  VICINITY  OF  THE  MALLMAN  CAMPS 

Scale 
q M ii 1  mile 


Contour  interral  20  feet 


1 


i 


PLATE   VII. 


MON   XLIII — 03 6  81 


PLATE   VII. 

PHOTOMICROGRAPHS  OF  NORMAL  AND   METAMORPHOSED   LOWER  HUROXIAN   GRAYWACKE. 

Fig.  a. — Lower  Huronian  graywacke.  Specimen  45412,  slide  15697.  From  1,300  paces  north  of 
west  of  the  southeast  corner  of  sec.  16,  T.  59  N.,  R.  14  W.  With  analyzer,  x  50.  This  is  the  normal 
phase  of  Lower  Huronian  graywacke,  consisting  of  quartz  and  feldspar  grains,  mainly  the  former, 
very  imperfectly  rounded,  and  a  considerable  amount  of  secondary  biotite  and  muscovite  or  sericite. 
All  the  constituents  have  a  dimensional  parallelism,  and  the  micas  have  also  a  crystallographic 
parallelism.     Described  pp.  74-75. 

Fig.  B. — Lower  Huronian  graywacke.  Specimen  45414,  slide  15700.  From  1,680  paces  north  of 
west  of  the  southeast  corner  of  sec.  16,  T.  59  N.,  R.  14  W.  With  analyzer,  x  50.  This  is  nearer  the 
intrusive  granite  contact  than  the  specimen  shown  in  A,  and  shows  a  more  abundant  development 
of  the  secondary  minerals  and  a  coarsening  of  the  grain.  The  grain  is  coarser  and  more  irregular, 
due  to  the  recrystallization  of  the  quartz  and  feldspar.  Abundant  biotite  and  green  hornblende  have 
developed;  muscovite  is  nearly  lacking.  Abundant  accessories  are  magnetite,  ilmenite,  rutile,  sphene, 
and  garnet.  The  rutile  may  be  seen  surrounded  by  and  altering  into  sphene  (titanomorphite). 
Described  pp.  83-84. 

Fig.  C. — Lower  Huronian  graywacke.  Specimen  45415,  slide  15701.  From  near  the  southeast 
corner  of  sec.  9,  T.  59  N.,  R.  14  AV.  AVith  analyzer,  x  50.  This  is  still  nearer  the  granite  contact  than 
the  specimens  figured  as  A  and  B,  and  shows  correspondingly  coarser  crystallization  and  more  abundant 
development  of  secondary  minerals.  The  hornblende  is  the  dominant  secondary  constituent,  and  biotite 
is  almost  lacking.     The  same  accessory  minerals  are  present  as  in  fig.  B.     Described  pp.  83-84. 

Fig.  B. — Lower  Huronian  graywacke.  Specimen  45416,  slide  15703.  From  near  the  east  quarter 
post  of  sec.  9,  T.  59  N.,  R.  14  AV.  AA'ith  analyzer,  x  50.  The  slide  is  cut  from  within  an  inch  of  the 
granite  contact  and  shows  the  coarse  recrystallization  and  abundant  development  of  secondarj'  min- 
erals in  the  Lower  Huronian  graywacke.  The  feldspar  shows  cloudy  alterations,  and  the  quartz  shows 
undulatory  extinction.  The  dark  mineral  is  almost  entirely  fresh,  green  hornblende.  The  accessories 
are  sphene,  rutile,  ilmenite,  and  epidote.  If  this  rock  were  found  by  itself  and  not  connected  by 
gradations  with  normal  graywacke  and  slate  it  could  not  be  recognized  as  a  derivative  of  a  sedimentary 
rock.  Described  pp.  83-84. 
82 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XLIII   PL.  VII 


PHOTOMICROGRAPHS  SHOWING   PROGRESSIVE  METAMORPHISM   OF  LOWER   HURONIAN 
GRAYWACKE   IN   APPROACHING   INTRUSIVE  GRANITE. 


THE   MERIDEN   SRAVURE   CO. 


THE  LOWER  HURONIAN  SERIES.  83 

VEIN    QUARTZ. 

In  both  the  Lower  Huronian  sedimeutar}'  and  granitic  rocks,  particu- 
larly the  former,  there  are  abundant  veins  of  quartz,  resulting  from  infiltra- 
tion along  joints  and  brecciated  zones.  This  vein  quartz  has  yielded 
numerous  and  conspicuous  pebbles  to  the  conglomerates  at  the  base  of  the 
overlying  Upper  Huronian  series. 

At  the  contact  of  the  granite  and  the  Lower  Huronian  series,  also,  there 
has  been  a  segregation  of  quartz  in  irregular  veins  and  stringers,  and  in  this 
case  it  is  believed  that  such  quartz  was  in  part  deposited  from  hot  solutions 
accompanving  the  intrusion  of  the  granite. 

META3IORPHISM   OF   LOWER   HURONIAIV   ROCKS   BY   GRANITE. 

The  intrusion  of  the  granite  above  described  has  further  greatly  meta- 
morphosed the  graywackes  and  slates,  which  are  themselves  the  altered 
equivalents  of  muds  and  sands.  In  approaching  the  granite  they  become 
more  chloritic,  hornblendic,  and  micaceous,  and  a  marked,  and  usually 
much  contorted,  schistosity  obliterates  the  bedding.  They  become,  in 
short,  chloritic,  hornblendic,  and  micaceous  schists.  The  planes  of  parting 
have  Colors  characteristic  of  chlorite,  hornblende,  and  mica,  and  when 
weathered  not  infrequently  exhibit  silvery  and  bronzy  lusters.  Under  the 
microscope  the  rock  may  be  seen  to  have  undergone  extensive  alteration. 
There  has  been  abundant  development  of  secondary  chlorite  and  horn- 
blende and  a  lesser  development  of  secondary  biotite  and  muscovite. 
Abundant  accessories  characteristic  of  metamorphic  rocks  of  this  nature 
are  present.  They  include  tourmaline,  staiirolite,  g-arnet,  rutile,  ilmenite, 
'  magnetite,  and  apatite.  The  alteration  of  the  ilmenite  and  rutile  to  sphene 
(titanomorphite)  is  well  exhibited.     (Specimen  45414.) 

It  is  noticeable  that  the  development  of  secondary  minerals  is  greater 
in  rocks  showing  more  feldspar  and  less  in  rocks  consisting  mainly  of  quartz, 
as  would  be  expected.  Accompanying  this  development  of  new  minerals 
there  has  been  a  recrystallization  of  the  original  quartz  and  feldspar,  which 
has  resulted  in  increasing  the  size  of  the  grains  and  in  obliterating  all  evi- 
dence of  their  clastic  character  as  well  as  of  bedding.  Rarely,  also,  the 
quartz  particles  have  been  made  to  lie  with  their  principal  axes  parallel 
to  the  schistosity,  thus   showing   crystallographic  as  well  as   dimensional 


84  THE  MESABI  IRON-BEARING  DISTRICT. 

parallelism.  (Specimen  45492.)  In  the  most  altered  phases  feldspar 
crystals,  which  are  several  times  the  size  of  any  found  in  the  unaltered 
graywackes  or  slates,  and  which  have  somewhat  irregular  outlines,  stand 
out  among  smaller  quartz  grains,  and  are  larger  than  any  of  the  feldspars 
in  the  unaltered  graywackes.  Following  the  recrystallization  there  has 
been  a  considerable  cloudy  alteration  of  the  feldspar.  Where  the  original 
rock  was  mainly  quartz  the  grain  of  the  metamorphic  equiA-alent  is 
uniformly  finer  than  the  grain  of  the  rock  originally  strongly  feldspathic. 
Between  and  around  the  quartz  and  the  feldspar  are  abundant  fresh 
secondary  chlorite  and  hornblende  and  less  abundant  mica,  in  general 
roughly  parallel,  but  in  detail  following  the  peripheries  of  the  quartz  and 
feldspar  grains.  While  the  highly  developed  schistose  structure  shows 
that  the  rocks  have  undergone  great  compression,  none  of  the  mineral 
constituents  show  any  strain  effects  whatever  because  of  the  complete 
recrystallization. 

Starting  at  some  little  distance  from  the  granite  contact,  the  graywackes 
and  slates  are  of  the  kind  above  described  as  normal  for  the  formation.  In 
approaching  the  contact  the  metamorphic  features  just  described  become 
more  and  more  evident,  until  we  find  their  typical  development  imme- 
diately at  the  contact.  Were  it  not  for  the  complete  gradation  it  would 
not  be  possible  from  the  character  of  the  rocks  to  show  that  the  highly 
altered  schists  near  the  granite  are  really  of  sedimeiitary  origin  and  the 
metamorphosed  equivalents  of  the  graywackes  and  slates.  The  series  of 
photomicrographs  (PI.  VII)  show  how  the  microscopic  aspect  of  the  gray- 
wackes and  slates  changes  in  approaching  the  granite. 

The  hornblendic  graywackes  associated  with  the  Archean  hornblendic 
schists  north  of  Mountain  Iron  and  Hibbing  correspond  in  all  essential 
features  with  the  hornblendic  graywackes  formed  liy  the  contact  of  the 
granite. 

THE    RET^ATIONS     OF    LOWER    HURONIAN    GRAIS^ITE    TO    SEDIMENTS, 
AND   RELATIONS   OF   BOTH   TO   OTHER   SERIES. 

The  granites  are  throug-hout  intrusive  into  the  Lower  Huronian  sedi- 
ments.  Actual  intrusive  contacts  are  to  be  observed  in  a  number  of  places. 
The  Lower  Huronian  shows  the  metamorphic  effects  of  the  intrusion,  and 
near  the  contacts  no  conglomerates  are  to  be  observed.  The  contact  of  the 
"■ranite    and    Lower    Huronian    sediments   is   well    exposed    northwest    of 


THE  LOWER  HCRONIAN  SERIES. 


85 


Mesaba  station  in  the  SE.  J  of  SE.  i  sec.  18,  T.  59  N.,  R.  14  W.,  and 
northeast  of  Mesaba  station  50  steps  north  of  the  east  quarter  post  of  sec. 
9,  T.  59  N.,  R.  14  W.  Near  the  contact  at  the  former  place  the  graywacke 
is  shot  through  and  through  with  stringers  of  granite.  The  ahernating 
layers  of  graywacke  and  granite  in  some  instances  vary  from  a  fraction  ol 
an  inch  up  to  several  feet.  In  general,  the  injection  with  the  granite  has 
been  parallel  to  the  planes  of  schistosity  in  the  altered  graywacke,  but  in  a 
number  of  places  the  granites  may  be  seen  cutting  across  the  schistosity. 
The  sketch  (fig.  2)  shows  the  intricacy  of  the  contact  at  this  place.  The 
contact  effect  of  the  granite  on  the  sediments  has  already  been  described. 
The  granite  itself  close  to  the  contact  becomes  very  fine  grained,  but 
otherwise  does  not  differ  essentially  from  tlie  granite  of  the  main  mass. 


Fig.  2.— Sketch  of  contact  of  Lower  Huronian  granite  and  graywacke-slate,  showing  intricate  nature  of  granite  Intrusion. 

Another  even  more  complex  contact  may  be  observed  north  of 
Mountain  Iron,  northwest  of  the  center  of  sec.  34,  near  the  Archean- 
Upper  Huronian  boundary. 

While  the  evidence  is  conclusive  that  the  great  mass  of  the  granite  is 
intrusive  into  the  Lower  Huronian,  it  is  not  at  all  certain  that,  for  limited 
areas,  the  granites  here  mapj^ed  and  described  as  Lower  Huronian  may  not 
contain  granite  of  later  date.  The  granites  show  great  lithologic  com- 
plexity, and  where  the  points  affording  evidence  of  relation  are  as  widely 
separated  as  they  are,  particularly  in  the  western  portion  of  the  range,  parts 
of  the  granite  may  be  intrusive  into  the  main  granite  mass  and  thus  per- 
haps be  of  post-Lower  Huronian  date,  and  still  no  evidence  of  this  appear. 
Because  of  the  lack  of  exposures,  it  is    not  unlikely  that  even  the  most 


86  THE  MESABI  IRON-BEARiNG  DISTRICT. 

detailed  field  work,  with  this  point  alone  in  mind,  would  fail  to  delimit 
the  later  g-ranites. 

The  conglomerate  forming  the  great  part  of  Lower  Hiironian  sedi- 
ments afiPords  conclusive  proof  that  the  Lower  Huronian  sediments  rest 
unconformably  upon  the  Archean  rocks.  Every  kind  of  pebble  found  in 
this  conglomerate,  with  the  possible  exception  of  a  few  cherty  slate 
pebbles,  can  be  matched  among  the  Archean  rocks.  The  conglomerate 
can  best  be  studied  in  the  cut  in  the  Duluth  and  Iron  Range  track  east  of 
Mariska,  in  the  NE.  i  of  NE.  i  sec.  22,  T.  58  N.,  R.  17  W.,  and  northwest  of 
Biwabik,  in  the  SW.  i  of  SW.  i  sec.  34,  T.  59  N.,  R.  16  W.  At  the  latter 
place  the  actual  contact  of  the  two  formations  can  be  observed  and  the 
conglomerate  at  the  base  of  the  Lower  Huronian  contains  pebbles  identical 
with  the  adjacent  Archean  igneous  rocks. 

Both  the  Lower  Huronian  sediments  and  granites  are  unconformably 
underneath  the  Upper  Huronian  series,  as  shown  both  by  structure  and  by 
conglomerates  at  the  base  of  the  Upper  Huronian  sediments.  This  uncon- 
formity is  described  in  connection  with  the  Upper  Huronian  series. 

STKUCTLrRE. 

The  Lower  Huronian  sedimentary  series  shows  a  conspicuous  sedi- 
mentary bedding.  The  area  has  been  so  folded  that  the  beds  now  stand 
on  edge,  the  dip  seldom  varying  more  than  5°  or  10°  from  vertical. 
Superposed  upon  the  original  bedding  structure  is  an  excellent  secondary 
cleavage.  The  cleavage  planes,  for  the  most  part,  are  approximately 
parallel  to  the  bedding  planes.  The  strike  of  both  bedding  and  cleavage 
is  uniform,  about  N.  60°  E.,  although  locally  var}-ing  10°  to  20°  from  this 
direction. 

Both  the  Lower  Huronian  sediments  and  the  granites  are  jointed, 
the  sediments  particularly  so.  The  sediments,  moreover,  show  conspicuous 
faulting-  and  brecciation.  These  features  may  be  well  observed  425 
paces  west,  275  paces  south  of  the  northeast  corner  of  sec.  32,  T.  58  X., 
R.  17  W.,  and  just  south  of  the  northwest  corner  of  sec.  3,  T.  58  N.,  R. 
16  W.  The  breccias  at  these  places  might  be  mistaken  for  conglomerate, 
especially  as  at  the  latter  place  there  is  also  present  a  small  amount  of 
true  conglomerate  (see  p.  96),  but  they  are  believed  to  be  breccias,  for  the 
following  reasons:    (1)  The  fragments  are  identical  with  the  material  of  the 


THE  LOWER  HURONIAN  SERIES.  ,  87 

strata  adjacent.  (2)  The  fragments  are  angular;  certain  quartz  fragments 
are  rounded,  but  the  rounding  is  due  to  the  pinching  out  of  quartz  veins; 
intermediate  steps  of  the  process  are  to  be  observed.  (3)  The  interstitial 
material  is  largely  vein  quartz.  (4)  Finally,  the  so-called  breccias  occur  in 
definite  vertical  zones,  striking  almost  north  and  south;  that  is,  almost 
directly  across  the  sedimentary  bedding.  The  supposed  breccia  grades 
into  the  unbroken  strata,  which  have  a  normal  strike  on  each  side.  More- 
over, in  attempting  to  match  the  beds  on  different  sides  of  the  brecciated 
zones  it  is  found  that  there  has  been  faulting,  oftentimes  as  much  as  several 
feet. 

THICKIfESS. 

As  the  bedding  stands  directly  on  edge,  the  width  of  the  formation 
across  the  strike  may  measure  the  thickness  of  the  series.  On  this  basis 
the  thickness  may  amount  to  7,000  feet.  However,  the  beds  may  repre- 
sent limbs  of  a  closely  compressed  fold,  or  j)erhaps  several  folds,  which 
have  been  truncated,  and  in  such  a  case  the  apparent  thickness  of  the  series 
is  much  greater  than  the  true  thickness.  Large  areas  of  the  formation  are 
not  exposed,  and  while  there  is  no  positive  evidence  of  duplication  of  beds 
the  probability  is  that  they  are  duplicated.  In  view  of  this  probability, 
3,000  to  5,000  feet  is  probably  as  great  a  thickness  as  can  safely  be 
assigned  to  the  Lower  Huronian  sediments  of  the  Mesabi  district. 


CHAPTER  V. 

THE  UPPER  HURONIAN  SERIES. 

The  sedimentary  rocks  of  Upper  Huroniau  age  occup}^  practically  all 
the  southern  slopes  of  the  range  from  one  end  of  the  district  to  the  other, 
and  extend  also  an  unknown  distance  south  beneath  the  g'lacial  diift.  The 
surface  width  of  the  series  in  the  ai'ea  included  in  the  district  described 
varies  from  less  than  1  mile  to  5  miles  or  more.  The  beds  of  the  series 
have  a  flat  dip  to  the  south.  Their  upper  edges  being  truncated,  they 
appear  in  belts  winding  along  parallel  to  the  range,  the  northerly  belts 
representing  the  lower  beds  and  the  southerly  belts  the  higher  beds  of  the 
series. 

The  exposures  of  the  Upper  Huronian,  particularly  on  the  lower 
slopes,  are  so  widely  separated  that  the  mapping  of  the  series  would  have 
been  an  impossibility  had  it  not  been  for  numerous  test  ^jits  sunk  in  search 
for  ore,  whicli  were  bottomed  in  the  Upper  Huronian  series.  These  are 
particularly  numerous  along  the  central  portion  of  the  range,  and  have 
enabled  the  distribution  of  the  Upper  Huronian  rocks  to  be  indicated 
within  rather  ("lose  limits  for  this  part  of  the  range. 

The  U]:)per  Huronian  series  comprises  from  the  base  up  (1)  the 
Pokegama  formation,  consisting  mainly  of  quartzite,  but  containing  also 
conglomerate  at  its  base;  (2)  the  Biwabik  formation,  consisting  of 
ferruginous  cherts,  iron  ores,  slates,  greenalite  rocks,  and  carbonate  rocks, 
with  a  small  amount  of  coarse  detrital  material  at  its  base;  and  (3)  the 
Virginia  slate.  Between  the  Pokegama  quartzite  and  the  Biwabik  forma- 
tion there  is  a  slight  erosion  interval.  Tlie  Biwabik  forniati(M\  grades 
conformably  into  the  Virginia  slate  both  vertically  and  laterally.  In  all 
previous  geologic  work  on  the  district  the  detrital  rocks  forming  the 
base  of  the  iron  formation  (quartzite  and  conglomerate)  have  been  con- 
sidered a  part  of  the  Pokegama  formation,  the  presence  of  the  slight 
break  between  such  detrital  rocks  and  the  underlying  Pokegama  formation 
having  been  overlooked.     On  tlie  iiccoiupanx  iug  geologic  map,  also,  the 

88 


THE  UPPER  HURONIAN  SERIES. 


89 


basal  detritals  of  the  iron  formation  have  been  included  in  the  Pokegama 
qnartzite.  This  is  done  for  the  reasons  that:  (1)  The  layer  of  basal  iron- 
formation  fragmentals  for  the  most  part  is  so  thin  that  it  can  not  be 
indicated  on  the  general  map  without  exaggeration;  (2)  in  much  of  the 
district  exploration  has  not  been  sufficient  near  the  boundary  of  the  iron 
formation  to  allow  of  the  discrimination  of  the  quartzite  and  conglomerate 


R.  17  W 


R.  17  W. 


Scale 


LEGEND 

ALGONKIAN 


Ferruginous  chert 

of  BiwabikCiron- 

bearingOformation 


Abq 


Quartzite  of  Eiwabit 

(iron-bearing) 

formation 


Apq 


Pokegama  quartzite 


Outcrop 


Fig. 


0  X  JimUe 

Contour  interval  20  feet 

-Detail  map  of  sec.  3,  T.  58  N.,  E.  17  W.,  showing  separation  of  quartzite  at  the  base  of  the  Biwabik  formation  from 
the  Pokegama  quartzite.     f  A  correction  of  this  map  is  shown  on  PI.  II.) 


of  the  iron  formation  from  that  belonging  with  the  Pokegama  formation; 
(3)  for  economic  purposes  it  is  more  desirable  to  indicate  the  boundary 
between  the  possible  iron-bearing  area  and  quartzite,  regardless  of  the 
formation  to  which  the  latter  belongs,  than  between  the  two  structural 
geologic  units.  In  order  to  show  that  discrimination  between  the  Poke- 
gama quartzite  aud  the  detrital  material  at  the  base  of  the  iron  forma- 
tion is  possible  where  exploration  has  been  sufficient,  a  detailed  map  has 


90  THE  MESABI  IRON-BEARING  DISTRICT. 

been  made  of  a  part  of  sec.  3,  T.  58  N.,  R.  17  W.  (fig.  3),  where  the 
relations  of  the  Pokegama  quartzite  and  the  iron  formation  were  first 
satisfactorily  worked  out.  "While,  for  the  reasons  stated  above,  all  the 
detrital  material  at  the  base  of  the  iron  formation  has  been  included  in  the 
Pokegama  formation  on  the  general  map  (PI.  II),  the  description  of  the 
Biwabik  formation  on  a  subsequent  page  includes  the  detrital  material 
belonging  with  it. 

SECTIO^r  I.    THE  POKEGAMA  QUARTZITE. 

DISTRIBUTION. 

The  Pokegama  quartzite  is  the  basal  formation  of  the  Upper  Hurouian 
series.  Because  of  the  southerly  dip  and  truncation  of  the  series,  the 
quartzite  appears  as  a  belt  immediately  south  of  and  contiguous  to  the 
Lower  Huroniau  and  Archean  formations.  The  belt,  varying  from  a  few 
steps  to  a  half  mile  or  more  in  width,  extends  from  the  west  end  of  the 
Mesabi  district  continuously  to  north  of  Mountain  Iron.  From  here  on  to 
the  east  end  of  the  range  data  are  insufficient  for  mapping  the  quartzite  as 
a  continuous  belt,  and  it  is  accordingly  mapped  as  a  number  of  discontinuous 
areas  of  varying  width  and  length.  It  is  possible  that  future  exploration 
w  ill  result  in  extending  and  connecting  some  of  these  areas,  but  it  is  also 
certain  that  some  of  them  are  really  cut  ofP  from  one  another  because  of 
the  overlapping  of  the  iron  formation.  The  typical  Pokegama  quartzite  is 
exhibited  in  exposures  at  Pokegama  Falls,  on  the  Mississippi  River;  at 
Prairie  River  Falls  (fig.  4),  north  of  the  Arcturus  mine  in  sec.  13,  T.  56 
N.,  R.  24  W.;  north  and  northwest  of  Hibbing;  near  the  south  quarter 
post  of  sec.  20,  T.  53  N.,  R.  17  W.;  at  the  east  end  of  the  range  in  sees. 
29  and  32,  T.  60  N.,  R.  13  W.;  and  test  pits  and  drill  holes  have  been 
bottomed  in  the  quartzite  at  many  intermediate  points. 

KINDS    OF   ROCKS. 

The  Pokegama  formation  comprises  vitreous  quartzites  of  various  colors 
and  textm'es,  micaceous  quartz  slates,  and  conglomerates. 

QUARTZITE. 

The  bulk  of  the  Pokegama  formation  is  a  vitreous  quartzite.  Bedding 
is  well  marked  by  alternating  light  and  dark  bauds,  and  rarely  ripple  marks 
may  be  oljserved.     The  quartz  grains  are  well  rounded,  of  medium  size, 


THE  POKEGAMA  QUARTZITE. 


91 


and  in  g'eneral  are  bettei-  rounded  and  coarser  than  in  the  Lower  Huronian 
graywackes.     The  colors  are  dark  green,  grayish  green,  light  yellow,  or 


Quartzite 

25 o  .^75  feet 


Fig.  4. — Sketch  showing  distribution  of  Biwabik  and  Pokegama  formations  at  the  lower  falls  of  Prairie  Eiver. 


92  THE  MESABI  IRON-BEARING  DISTRICT. 

various  shades  of  red  and  brown.  In  some  cases  at  least  the  original 
colors  have  been  yellowish  and  grayish  green,  and  the  red  and  brown  colors 
have  resulted  from  the  infiltration  of  ferric  iron  and  from  the  oxidation  of 
ferrous  compounds  in  the  matrix  of  the  quartzite.  At  Pokegaraa  Falls  the 
quartzite,  for  some  feet  from  the  surface,  has  a  reddish  color,  but  where 
blasted  out  near  the  dam  the  red  colors  are  seen  to  give  wav  a  few  feet 
from  the  surface  to  the  grayish  or  yellowish  ones.  In  cracks  and  cre^aces 
the  iron  staining  has  penetrated  much  deeper.  In  other  rocks  the  yellow  or 
red  colors  are  due  to  numerous  grains  of  iron  oxide,  mainly  limonite,  associ- 
ated with  the  quartz  grains.  Weathering  has  not  only  discolored  the 
quartzite,  but  has  caused  it  to  disintegrate  to  a  certain  extent  by  softening 
the  cementing  material.  This  phenomenon  may  also  be  observed  at  Poke- 
gama  Falls. 

Under  the  microscope  the  quartzite  is  seen  to  be  made  up  of  well- 
rounded,  sometimes  subangular,  grains  of  quartz  and  an  occasional  grain  of 
microcline  feldspar.  In  the  proportion  of  quartz  and  feldspar  the  quartzite 
differs  from  the  Lower  Huronian  graywackes,  in  which  the  feldspar  and 
quartz  are  in  al^out  equal  abundance.  The  fragmental  grains  show  at 
places  consideral^le  effects  of  pressure  by  their  undulatory  extinction  and 
cracking,  but  seldom  are  these  effects  conspicuous.  More  frequently  are 
the  grains  cloudy,  due  to  the  inclusion  of  minute  specks  of  other  minerals. 
It  is  a  noticeable  fact  that  the  clastic  grains  in  the  finer  quartzites  are  less 
well  rounded  than  in  the  coarser  ones. 

Among  the  quartz  grains  there  appears  here  and  there  a  granule  of 
iron  oxide,  mainly  limonite,  which  in  some  cases  seems  to  have  partially 
or  wholly  replaced  quartz  grains  and  in  others  is  mixed  with  a  greenish  or 
yellowish  chloritic  substance  in  such  a  way  as  to  suggest  that  it  may 
have  replaced  a  granule  of  some  mineral  containing  ferrous  silicate.  The 
appearance  and  distribution  of  some  of  the  iron  oxide  grains  strongly  sug- 
gest their  development  tin-ough  the  alteration  of  iron  jiyrites  in  an  originally 
pyritiferous  quartzite,  but  while  such  development  is  probable,  no  direct 
evidence  of  it  has  been  observed. 

The  cementing  material  may  be  (1)  quartz  which  has  grown  out  in 
optical  continuity  with  the  original  grains,  containing  abundant  iron  oxide 
and  chloritic  discolorations,  or  (2)  a  confused  aggregate  of  greenish  material 
wliifli,  with  a  high  power,  is  found  to  be  mainly  chlorite,  with  subordinate 
amounts  of  actinolite  or  griinerite,  (juartz,  and  feldspar.     The  green  matrix 


THE  POKEGAMA  QUARTZITE.  93 

may  be  so  abundant  as  to  make  the  rounded  clastic  quartz  grains  stand  out 
from  it  like  phenocrysts,  or  it  may  be  sparse  and  give  way  to  the  discol- 
ored quartz  cement  formed  by  the  enlargement  of  the  quartzite  grains. 
The  oxidation  of  the  ferrous  iron  in  the  chloritic  cement  in  certain  of  the 
rocks,  particularly  the  weathered  ones,  has  yielded  red  and  brown  hematite, 
which  has  imparted  to  the  cement  a  red  or  brown  color.  In  other  rocks 
iron  oxide  has  been  infiltrated  from  above,  discoloring  the  cement  in  the 
same  way.  Comparing  the  slides  with  the  hand  specimens,  it  is  seen  that 
the  color  of  the  quartzite,  as  would  be  expected,  is  due  to  the  nature  of  the 
cementing  material.  Where  the  cement  is  mainly  quartz  formed  by  the 
enlargement  of  the  clastic  grains  of  quartzite  the  rock  is  one  of  the  lighter 
colored  gray  or  yellow  ones.  Where  the  cement  is  an  abundant  chloritic 
substance  the  quartzite  is  dark  gray  or  dark  green.  Where  considerable 
hematite  has  been  infiltrated  or  developed  by  alteration  the  quartzite  has 
distinctly  reddish  or  brownish  colors. 

The  dark-green  and  dark-gray  quartzites,  with  an  abundant  chloritic 
matrix,  are  similar  in  general  aspect  to  graywackes,  but  the  clastic  feldspar 
grains  are  too  few  to  wai-rant  the  application  of  this  name.  The  chloritic 
cement  in  the  quartzite  has  been  particulai-ly  studied  because  of  its  close 
resemblance  to  the  green  ferrous  silicate  occurring  in  granules  in  the  over- 
lying iron  formation.  It  is  indeed  possible  that  a  very  small  part  of  the 
material  here  called  chlorite  may  in  reality  be  a  ferrous  silicate  corres|)ond- 
ing  to  that  described  in  the  iron  forinatiou  on  a  subsequent  page,  but  no 
positive  evidence  of  this  has  been  found,  and,  on  the  contrary,  positive 
evidence  of  the  chloritic  nature  of  most  of  it  is  at  hand. 

Where  the  Pokegama  quartzite  is  in  direct  contact  with  the  basic 
igneous  rocks  of  the  Archean  it  takes  on  a  character  different  from  that 
noi-mal  to  the  formation.  In  sec.  33,  T.  59  N.,  R.  15  W.,  for  instance,  the 
particles,  instead  of  being  well-rounded  quartz  grains,  are  complex  grains 
derived  from  the  breaking  down  of  the  fine-grained  underlying  basalt,  and 
both  fragments  and  matrix  are  much  discolored  b}'  green  chloritic  and 
hornblendic  alteration. 

MICACEOUS    QUAETZ-SLATE. 

Closely  associated  with  and  interbedded  with  the  massive  quartzites 
above  described  are  thin-bedded  slaty  quartzites,  or  quartzitic  slates,  with 
a  considerable  amount  of  mica  and  excellent  parting  along  bedding  planes. 


94  THE  MESABI  IRON-BEARING  DISTRICT. 

They  are  very  dififerent  in  appearance  from  many  of  the  quartzites,  and  were 
they  not  actnally  observed  to  grade  into  and  to  be  conformably  bedded  with 
the  quartzites  at  a  number  of  places  they  would  scarcely  be  referred  to  the 
same  formation  on  lithologic  grounds.  They  usually,  though  not  always, 
overlie  the  massive  quartzite.  This  may  be  well  observed  just  northeast  (jf 
Prairie  River  Falls  (see  fig.  4)  and  just  southwest  of  the  center  of  sec  18, 
T.  59  N.,  R.  1 4  W.  The  color  varies  from  dark  greenish  gray  to  a  light  yellow 
or  pink  or  red.  The  red  and  pink  colors  are  frequently  due  to  the  infiltra- 
tion of  iron  oxide  from  above  along  bedding  planes.  The  texture  varies  from 
that  of  a  medium-  grained  quartzite  to  that  of  a  rather  coarse  shale.  The 
conspicuous  features  of  the  rocks  are  their  excellent  parting  parallel  to  bed- 
ding planes  and  the  mica  plates  on  the  parting  plane.  For  the  most  part 
the  parting  planes  are  smooth,  with  slight  ridges  roughly  resembling  fine 
ripple  marks,  bnt  not  uncommonly  they  are  somewhat  rough  and  contorted. 
On  these  surfaces  the  mica  does  not  form  a  continuous  layer,  but  appears 
in  separated  plates,  each  with  its  own  twinkling  reflection. 

Under  the  microscope  the  quartz-slates  are  seen  to  be  finer  grained 
than  the  quartzites  described  above.  The  grains  are  more  angular;  the 
interstitial  chloritic  aggregate  is  more  uniformly  present;  evidence  of 
enlargement,  while  present,  is  not  so  conspicuous  as  in  the  quartzite;  and, 
finally,  the  quartz-slates  possess  mica,  while  the  quartzites  do  not.  The 
frao-mental  grains  in  the  quartz-slate  are  mainly  quartz  and  rarely  microcline, 
as  in  the  quartzite,  and  the  green  chloritic  material  is  of  the  same  nature. 
Only  rarely  are  the  effects  of  pressure  to  be  observed.  The  mica  is  in 
separate  flakes,  with  their  greater  diameters  parallel  to  the  bedding,  as  are 
also  the  longer  diameters  of  the  quartz  grains.  It  resembles  in  its  occur- 
rence the  clastic  mica  plates  sometimes  seen  in  a  sedimentary  rock  demon- 
strably unaltered.  In  a  few  cases  where  the  quartz-slate  gives  evidence  of 
having  undergone  much  squeezing  and  alteration  the  mica  is  much  more 
abundant  and  clearly  secondary,  and  in  such  cases  it  has  a  tendency  to 
form  continuous  layers  along  the  parting  planes  rather  than  to  occur  in 
isolated  flakes  with  definite  outlines. 

CONGLOMERATES. 

From  a  structural  standpoint  the  conglomerates  are  the  most  interesting 
rocks  of  the  Pokegama  formation.  They  form  a  very  irregular  layer  but  a 
few  feet  thick  at  the  base  of  the  quartzite. 


THE  POKEGAMA  QUARTZITE.  95 

In  the  SE.  J  of  SE.  J  sec.  18,  T.  59  N.,  R  14  W.,  the  conglomerate 
is  in  several  small  patches  overlj'ing  both  the  Lower  Huronian  graywacke 
and  slate  and  the  Lower  Huronian  g-ranite.  The  next  rocks  to  the  south 
are  of  the  iron  formation,  and  although  there  is  no  evidence  of  Pokegama 
quartzite  occurring  in  this  immediate  vicinity,  there  is  plenty  of  room  for 
it,  and  it  is  known  less  than  a  half  mile  to  the  northwest.  There  is  thus 
reason  to  believe  that  future  exploration  may  show  it  here,  but  whether  or 
not  the  quartzite  or  the  iron  formation  immediately  overHes  the  conglom- 
erate at  this  point,  the  conglomerate  is  basal  to  the  Upper  Huronian,  and 
thus  should  be  described  at  this  place.  A  few  steps  west  and  north  of  the 
southeast  corner  of  sec.  18  the  conglomerate  can  be  observed  in  a  thin  layer 
mantling  over  the  Lower  Huronian  slate  and  graywacke.  Neither  the  bed- 
ding of  the  conglomerate  nor  that  of  the  underlying  graywacke  or  slate  is 
clear,  but  so  far  as  any  structure  is  present  in  either  it  is  a  horizontal 
one  in  the  conglomei-ate  and  a  vertical  one  in  the  lower  series.  The 
pebbles  of  the  conglomerate  are  fairly  small,  sometimes  reaching  a  size 
of  2  or  3  inches,  but  commonly  being  an  inch  or  less.  The  most  con- 
spicuous fragments  are  white,  gray,  or  black  vein  quartz,  but  these  are 
scarcely  more  abundant  than  pebbles  of  graywacke  and  slate  identical  with 
those  of  the  underlying  Lower  Huronian  series.  With  these  abxindant 
pebbles  are  a  few  scattering  and  doubtful  pebbles  of  granite.  At  425  steps 
north  and  460  west  of  the  southeast  corner  of  sec.  18  is  again  a  thin  layer 
of  conglomerate  mantling  over  the  Lower  Huronian  granite.  Only  a  short 
distance  awa}^  the  granite  is  found  intrusive  in  the  Lower  Huronian  sedi- 
ments, and  hence  the  Upper  Huronian  age  of  the  conglomerate  is  unques- 
tionable. Here  the  conglomerate  contains  pebbles  and  bowlders  up  to  2 
feet  or  more  in  diameter,  consisting  of  graywacke  and  slate,  white,  gray,  or 
black  chert,  granite  identical  with  that  immediately  underlying,  and  in 
addition  pebbles  of  a  fine-grained  granite  similar  to  that  sometimes  seen  in 
dikes  in  the  Lower  Huronian  granite.  Certain  pebbles  of  doubtful  character 
may  represent  basic  ig-neous  rocks.  The  matrix  is  a  quartzite  in  which  the 
quartz  grains  are  well  rounded  and  set  in  a  dark  greenish-  or  purplish- 
black  matrix,  discolored  by  chloritic  substances.  In  general  the  conglom- 
erates described  in  this  area  contain  a  typical  assemblage  of  pebbles  and 
bowlders  representing  the  various  phases  of  rock  found  in  the  immediately 
underlying  Lower  Huronian. 


96  THE  MESABI  IRON-BEARING  DISTRICT. 

A  little  southwest  of  the  center  of  the  XW.  i  of  SW.  ^  sec  33,  T.  59 
N.,  R.  15  W.,  is  a  pit  which  has  passed  through  quartzite  and  conglomerate 
into  Archean  basalt.  The  conglomerate  is  a  much  metamorphosed  one, 
containing  basalt  pebbles  identical  with  the  basalt  below.  The  conglomerate 
is  thus  unconformably  upon  the  Archean,  and  is  basal  to  the  quartzite. 

Near  the  powder  house,  in  the  NW.  ^  of  NW.  J  sec.  3,  T.  58  N.,  R.  16 
W.,  is  a  thin  film  of  conglomerate  on  the  upper  svirface  of  the  southernmost 
exposure  of  Lower  Huronian  slate  and  graywacke.  The  next  rock  to  the 
south  is  the  Pokegama  quartzite.  The  conglomerate  is  composed  mainly 
of  vein  quartz  and  slate,  with  a  few  feldspar  porphyry  pebbles.  The  vein 
quartz  and  porphyry  pebbles  are  well  rounded,  while  the  slate  fragments 
are  angular.  It  is  believed  that  this  may  be  a  part  of  the  conglomerate  at 
the  base  of  the  Pokegama  quartzite.  However,  just  to  the  north  and  north- 
west, in  the  Lower  Huronian  area,  are  certain  breccias  (see  pp.  86-87) 
which  are  almost  identical  with  the  conglomerate  except  that  the  fragments 
are  not  so  well  rounded.  It  is  not  impossible  that  this  supposed  conglom- 
erate may  be,  in  part,  a  breccia,  although  much  of  it  certainly  is  not. 

On  the  road  a  little  north  and  a  little  east  of  the  northeast  corner  of  sec. 
3,  T.  58  N.,  R.  16  W.,  in  the  drainage  ditch  close  to  the  Biwabik  mine,  is 
an  obscure  conglomerate  in  contact  with  the  Archean  green  schists.  The 
rock  is  reddish,  and  the  fragments,  as  nearly  as  can  be  made  out,  are 
of  green  schistose  rocks  like  those  of  the  Archean  subjacent,  but  very 
niucli  altered.  The  rock  immediately  to  the  south  is  the  Pokegama 
quartzite. 

Just  north  of  Roberts  mine  at  McKinley  are  pits  which  have  passed 
through  Pokegama  quartzite  and  conglomerate  into  Lower  Huronian  gray- 
wacke and  slate.  The  conglomerate  at  the  base  of  the  Pokegama  contains 
pebbles  of  the  graywacke  and  slate  below. 

Noi'th  of  Mountain  Iron,  just  northwest  of  the  center  of  sec.  34,  T.  59 
K.,  R.  18  W.,  are  small  patches  of  cong-lomerate  intricately  mixed  up  with 
the  Archean  and  Lower  Huronian  rocks  which  come  together  at  this  ]ioint. 
(See  PI.  V.)  The  Upper  Huronian  rocks  occur  just  to  tlie  southeast 
across  a  little  valley.  The  older  rocks  show  much  intricacy  of  structure, 
erosion  lias  consequent!)'  cut  down  into  them  unequally,  and  finally  some 
of  the  contacts  are  drift  covered,  so  that  it  has  been  exceedingh*  difficult  to 
determine  the  ti'ue  relations  of  the  conglomerate  and  the  adjacent  rocks. 


I 
I 

i 


THE  POKEGAMA  QUARTZITE.  97 

However,  there  appear  to  be  here  Archean  hornblende-schists  and 
Lower  Hiirouian  graywackes  and  slates,  and  both  of  these  are  intruded 
by  Lower  Huronian  granite.  On  top  of  this  complex  are  patches  of  con- 
glomerate containiiag  well-rounded  pebbles  of  white  weathering  slate  and 
graywacke,  I'eaching  a  size  of  several  inches,  and  resembling  the  Lower 
Huronian  sediments  near  at  hand.  A  few  doubtful  pebbles  may  be  Archean 
igneous  rocks.  No  granite  pebbles  were  observed.  The  matrix  is  a  well- 
bedded  graywacke,  much  contorted,  of  dark-green  or  dark-gray  color, 
with  mica  plates  on  bedding  planes.  It  seems  likely  that  the  conglomerate 
is  basal  to  the  Upper  Huronian.  But  there  is  a  distinct  possibility  that  the 
conglomerate  after  all  is  basal  to  the  Lower  Huronian  sediments,  and  that 
the  pebbles  of  sediments  observed  are  from  the  Basement  complex.  This 
possibility  is  suggested  by  the  similarity  in  appearance  of  the  matrix  of  the 
conglomerate  to  the  typical  Lower  Huronian  graywackes  close  at  hand  and 
by  the  absence  of  Lower  Huronian  granite  pebbles. 

In  sees.  25  and  26,  T.  58  N.,  R.  21  W.,  north  of  Hibbing,  are  large 
bowlders  of  conglomerate  in  the  drift.  They  are  foiind  just  south  of  the 
northern  boundary  of  the  quartzite  and  were  undoubtedly  carried  there  by 
the  glaciers  from  this  boundary.  They  are  similar  to  the  conglomei-ates 
north  of  Mesaba  station,  their  principal  pebbles  being  graywacke  and 
slate,  vein  quartz,  and  granite,  and  in  addition  there  are  seen  fragments  of 
chlorite-schists  and  mica-schists  similar  to  these  rocks  in  the  Lower  Huron- 
ian and  Archean  areas  adjacent. 

On  the  west  line  of  sec.  i^4,  T.  58  N.,  R.  21  W.,  on  the  south  escarp- 
ment of  the  southernmost  exposure  of  Lower  Huronian  granite,  is  a  con- 
glomerate with  pebbles  up  to  4  or  5  inches  in  diameter  of  vein  quartz,  white, 
gray,  brown,  and  reddish,  and  of  granite  identical  with  that  of  the  solid 
ledge  underneath.    As  usual  the  fine  quartz  pebbles  ai-e  the  most  conspicuous. 

At  1,200  steps  south  and  400  west  of  the  northwest  corner  of  sec.  3, 
T.  56  N.,  R.  23  W.,  is  a  conglomerate  lying  on  the  eastward -facing  escarp- 
ment of  the  granite,  that  is,  between  the  granite  and  the  Pokegama 
quartzite.  The  pebbles  are  several  inches  in  diameter.  They  consist  of 
granite  identical  with  that  beneath,  of  vein  quartz  and  of  jasper.  The  vein 
quartz  is  very  abundant  and  forms  the  most  conspicuous  pebbles.  Certain 
of  the  pebbles  are  somewhat  discolored  by  iron.  One  ellipsoidal  pebble, 
with    a    greater  diameter  of  3    inches,    seems    to    be    a   true,  bright-red, 

MON  XLIII — 03 7 


98  THE  MESABl  IRON-BEARING  DISTRICT. 

slightly  bauded  jasper.  It  is  not  impossible  that  this  represents  a  very 
highly  iron-stained  phase  of  the  vein  quartz  so  conspicuous  in  the  pebbles, 
but  on  this  basis  it  is  difficult  to  account  for  the  banding.  A  careful  exam- 
ination in  the  laboratory,  both  microscopic  and  macroscopic,  leaves  little 
doubt  that  the  rock  is  a  true  jasper,  that  is,  one  of  the  phases  of  rock  com- 
monly associated  with  iron  ore.  Jasper  has  not  been  found  in  the  lower 
Huronian  or  Archean  of  this  district,  but  it  occurs  abundantly  in  this  series 
in  other  districts,  and  this  particular  joebble  may  have  come  from  a  very 
considerable  distance.  On  the  other  hand,  it  is  not  impossible  that  future 
exploration  may  show  small  areas  of  jasper  in  the  Lower  Huronian  of  this 
district. 

In  general,  throughout  the  range  the  conglomerates  which  can  safely 
be  assigned  to  the  base  of  the  Pokegama  quartzite  or  Upper  Huronian  vary 
only  in  relative  abundance  of  the  different  kmds  of  pebbles.  At  every 
locality  the  pebbles  are  predominantly  like  the  immediately  subjacent 
rocks.  The  striking  feature  of  the  conglomerates  throughout  is  their 
large  content  of  vein-quartz  pebbles.  This  feature  was  at  first  trouble- 
some, but  search  in  the  underlying  rocks  of  the  Lower  Huronian  has 
shown  abundant  vein  quartz  from  which  the  pebbles  could  be  derived. 
The  vein  quartz  being  hard,  massive,  and  homogeneous,  is  not  likely  to  be 
so  much  broken  up  as  the  granites  and  sediments  during-  the  time  it  is 
worked  over  by  water,  and,  for  the  same  reason,  fragments  become  better 
rounded.  The  few  chert  fragments  and  the  one  true  jasper  fragment  found 
in  the  conglomerate  have  not  yet  been  duplicated  in  the  underlying  rocks. 

STRUCTURE. 

The  Pokegama  quartzite  in  its  lower  and  middle  horizons  shows  but  a 
faint  bedding.  In  higher  horizons  the  bedding  is  well  shown  by  alternation 
of  light  and  dark  and  coarse  and  fine  bands,  parallel  to  which  is  an  excel- 
lent parting.  The  parting  planes  are  made  conspicuous  by  spangles  of 
clastic  mica.  As  a  part  of  the  Upper  Huronian,  the  Pokegama  quartzite 
has  participated  in  the  general  tilting  and  cross  folding  to  which  the  series 
as  a  whole  has  been  subjected,  and  thus  lies  with  a  gentle  flat  dip  to  the 
south,  with  gentle  cross  folds  whose  axes  are  transverse  to  the  range.  (See 
structure  of  the  Ujjper  Huronian,  pp.  178-180.)  Aside  from  the  tilting  and 
general  cross  folding,  the  Pokegama  quartzite  has  suffered  little  deforma- 


THE  POKEGAMA  QUARTZITE.  99 

tion.  Joints,  while  present,  are  inconspicnous,  and  little  or  no  secondary 
cleavage  has  been  developed  in  the  formation.  It  is  possible  that  there 
may  have  been  a  slight  amount  of  differential  movement  between  the  beds 
of  the  formation  due  to  the  gentle  folding,  and  this  may  account  for  a 
small  part  of  the  mica  seen  in  bedding  planes,  but,  as  already  noted,  it  is 
believed  that  a  greater  part  of  this  mica  is  clastic.  If  there  has  been 
much  movement  it  is  probable  that  there  would  have  been  a  greater 
development  of  secondary  mica  in  continuous  layers. 

THICKNESS. 

Because  of  the  few  exposm-es  and  the  difficulty  of  ascertaining  the 
variability  of  the  dip,  it  is  difficult  to  give  an  estimate  of  the  thickness  of 
the  formation.  Assuming  an  average  southward  dip  of  8  degrees,  and  the 
average  width  of  the  quartzite  belt  to  be  1,500  feet,  the  thickness  of  the 
formation  may  be  little  over  200  feet.  But  in  some  places  the  surface 
width  is  nearly  3,000  feet,  and  in  other  places  the  formation  is  cut  out 
entirely.  Moreover,  the  dip  varies  from  5  to  15  degrees.  The  thickness, 
therefore,  while  perhaps  averaging  about  200  feet,  may  vary  between  500 
feet  and  0.  E.  J.  Longyear  in  one  place  (1,025  paces  north  and  665  paces 
west  of  the  southeast  corner  of  sec.  35,  T.  58  N.,  R.  21  W.)  has  drilled 
from  the  iron  formation  completely  through  the  quartzite  and  found  it  to 
have  a  thickness  of  69  feet. 

RELATIONS   TO  OTHER  FORMATIONS. 

The  Pokegama  quartzite,  forming  as  it  does  the  base  of  the  Upper 
Huronian  series,  rests  unconformably  upon  the  Lower  Huronian  and 
Archean  series.  This  unconformity  is  considered  under  the  heading 
"Relations  of  the  Upper  Huronian  series  to  other  series"  (see  pp.  180-181). 
The  Pokegama  quartzite  is  overlain  by  the  iron  formation,  and  while  the 
two  formations  are  essentially  conformable  in  bedding  and  structure,  there 
is  between  the  two  a  thin  but  persistent  layer  of  conglomerate,  indicating 
a  minor  erosion  interval.  This  is  described  in  connection  with  the  iron- 
bearing  formation  (see  p.  154). 


100  THE  MESABI  IRON-BEARING  DISTRICT. 

SECTION  II.     THE  BIWABIK  FORIMATIOIV  (IROI^-BEARIIS^G). 

DISTRIBUTION. 

The  Biwabik  formation  extends  along  the  slopes  of  the  range  for  its 
entire  length,  from  west  of  Grrand  Rapids  to  Birch  Lake,  a  distance  of  nearly 
100  miles.  The  width  of  the  formation,  averages  perhaps  1\  miles,  bnt 
is  in  places  as  great  as  3  miles  and  in  others  as  small  as  a  quarter  of  a  mile. 
The  total  area  is  approximately  127  square  miles.  The  bounding  formation 
on  the  north  is,  for  the  most  part,  the  Pokegama  quartzite,  but  where 
this  is  lacking  the  Biwabik  formation  comes  in  contact  with  the  Lower 
Huronian  and  Archean  rocks.  To  the  south  the  iron-bearing  formation  is 
bounded  by  the  Virginia  slate,  except  in  range  12  and  a  part  of  range  13, 
at  the  east  end  of  the  range,  where  the  Duluth  gabbro  laps  np  over  the 
formation. 

On  account  of  the  covering  of  glacial  drift,  exposures  of  the  iron- 
bearing  formation,  except  in  the  eastern  end  of  the  district,  are  few. 
But  the  formation  has  been  reached  and  pierced  in  thousands  of  places  by 
drills  and  mining  excavations,  and  it  is  therefore  possible,  particularly 
along  the  part  of  the  range  at  present  productive,  to  delimit  the  iron  forma- 
tion with  a  fair  degree  of  accuracy.  In  parts  of  the  district  where  explo- 
rations and  mining  have  not  been  so  extensive,  especially  in  the  west  end 
of  the  district,  future  explorations  are  likel}"  to  show  that  the  boundaries, 
particularly  the  southern  boundary,  ai-e  in  some  localities  not  correct. 

The  iron  formation  in  general  occupies  the  middle  slopes  of  the  Giants 
range,  and  its  north  and  soutli  boundaries  have  fairly  uniform  altitudes  for 
considerable  distances.  By  an  examination  of  the  map,  however,  it  may 
be  seen  that  the  elevation  of  the  iron  formation  increases  from  the  west  end 
of  the  district  to  the  east,  the  total  difference  amounting  to  as  raiicli  as  500 
feet.  This  corresponds  with  the  increased  elevation  of  the  range  as  a  whole 
in  this  direction,  although  the  higher  elevation  of  the  southern  limit  of  the 
iron  formation  at  the  east  end  of  the  range  is  in  part  due  to  the  fact  that 
the  lower  parts  of  the  formation  are  overlapped  by  gabbro.  It  may  be 
further  seen  that  the  elevations  of  the  north  and  sontli  boundaries  show 
local  fluctuations  as  great  as  200  feet,  due  to  the  folding  of  the  formation 
;iiid  to  differences  in  depth  of  ei'osiou. 


THE  BIWABIK  FORMATION.  101 

KINDS  OF  ROCKS. 

The  great  bulk  of  the  Biwabik  formation  is  ferruginous  chert"  more  or 
less  amphibolitic,  calcareous,  or  sideritic  and  ^ray,  red,  yellow,  brown,  or 
green,  with  bands  and  shots  of  iron  ore.  It  is  analogous  to  the  jaspers  of 
the  other  iron  ranges  but  difPers  in  certain  particulars,  as  will  be  seen  on  a 
subsequent  page. 

Associated  with  the  chert,  mainly  in  the  middle  horizon,  are  the  iron 
ores.  Their  surface  area  is  only  about  5  per  cent  of  the  total  area  of  the 
iron-bearing  formation,  and  the  pi'oportion  of  their  bulk  to  that  of  the  iron- 
bearing  formation  is  much  less.  Near  the  bottom  of  the  Biwabik  forma- 
tion is  a  small  amount  of  conglomerate  and  quartzite — that  is,  coarsely  clas- 
tic sediments.  A  minute  conglomeratic  layer  has  also  been  observed  in  the 
Mahoning  mine,  in  about  a  central  horizon  of  the  formation.  In  thin  lay- 
ers and  zones  throughout  the  iron-bearing  formation,  and  particularly  in  its 
upper  horizons,  are  layers  of  slate  and  of  paint  rock,  the  paint  rock  usually 
resulting  from  the  alteration  of  the  slate.  Between  the  slate  and  the  paint 
rock  and  the  ferruginous  chert  are  numerous  gradational  varieties,  most  of 
which  come  under  the  head  of  ferruginous  slate.  Associated  with  the  slaty 
layers  in  the  iron  formation,  or  closely  adjacent  to  the  overlying  Virginia 
slate,  are  green  rocks  made  up  of  small  green  graniiles  of  ferrous  silicate 
which  are  here  called  greenalite.  It  will  be  shown  later  that  these  are  the 
original  rocks  from  which  most  of  the  other  phases  of  the  iron  formation, 
including  the  ores,  have  resulted  by  alteration.  Finally,  certain  calcareous 
and  sideritic  rocks  are  present  in  small  quantity,  particularly  near  the 
upper  horizons,  associated  with  the  greenalite  rocks.  The  rocks  of  the  iron 
formation  are  described  below,  beginning  with  the  original  type,  the  green- 
alite rock.     The  ores  are  reserved  for  a  separate  chapter. 

GREENALITE    ROCKS. 

Ill  limited  quantity  either  just  below  the  Virginia  slate,  or  associated 
with  some  slate  layer  in  the  iron  formation,  are  dull,  dark-green  rocks  of 
rather  uniform  fine  grain  and  with  conchoidal  fracture.     Layers  of  slate,  iron 

oThis  rock  has  been  called  taconite  by  the  geologists  of  the  Minnesota  survey,  and  the  name  has 
been  much  used  locally.  The  term  is  not  here  retained  for  the  reason  that  the  rock  ia  not  different 
from  ferruginous  cherts  of  other  parts  of  the  Lake  Superior  region,  as  described  in  the  monographs  of 
the  United  States  Geological  Survey,  and  there  is  no  reason  to  complicate  rock  nomenclature  by  add- 
ing a  new  name.     There  is  no  objection,  however,  to  its  local  use. 


102  THE  MESABl  IRON-BEARING  DISTRICT. 

ore,  aud  other  phases  of  the  iron  formation  usually  mark  their  bedding.  (See 
fig.  B,  PI.  VIIl.)  On  close  examination,  and  particularly  when  the  sur- 
face is  wet,  there  may  be  observed  numerous  ellipsoidal  granules  of  a  green 
substance  of  a  very  slightly  lighter  green  than  the  matrix  in  which  thev 
lie.  They  are  so  small  and  of  a  color  so  nearly  like  that  of  the  matrix  that 
they  are  likely  to  be  overlooked  unless  especiallj-  searched  for.  (See 
fig.  A' ,  PI.  VTII.)  An  occasional  one  is  of  much  greater  size  tlian  the 
average  and  looks  like  a  conglomerate  pebble  in  the  rock. 

Under  the  microscope  the  granules  are  conspicuous.  Their  cross 
sections  are  round,  oval,  in  some  cases  with  much  elongation,  crescent 
shaped,  lense  shaped,  gourd  shaped,  or  even  sharply  angular  (Pis.  IX,  XIII, 
XIV,  and  XV).  Here  and  there  a  curved  "tail"  seems  to  connect  one 
granule  with  its  neighbor  (PI.  IX).  Where  in  contact  with  a  layer  of  iron 
carbonate  or  calcium  carbonate,  as  they  frequently  are,  the  granules  become 
more  irregular  in  shape  and  project  into  or  are  included  in  the  carbonate 
layers  as  irregular  filaments  and  fragments.  The  carbonate  is  largely 
secondary  and  clearly  replaces  the  granules,  but  some  of  it  is  perhaps 
original,  and  in  this  case  the  variation  in  shape  of  the  granules  where 
associated  with  the  carbonate  layers  has  a  bearing  on  the  origin  of  the  ores, 
which  is  discussed  on  another  page.  One  hundred  and  twenty  measure- 
ments of  the  granules  show  an  average  greater  diameter  of  0.45  mm.  and 
average  least  diameter  of  0.21  mm.,  with  average  ratio  of  greatest  to  least 
of  100  to  47.  The  diameters  rarely  reach  1  mm.  and  seldom  drop  below 
0.1  mm.  Occasionally  certain  of  the  granules  may  be  seen  to  be  aggre- 
gated into  larger  granules^  with  well-rounded  outlines,  making  the  con- 
glomerate-like fragments  above  mentioned.  The  greater  diameters  of  the 
granules,  for  the  most  part,  are  parallel  to  the  bedding,  aud  in  fact  this 
arrangement  largely  detei-mines  the  bedding.  In  ordinary  light  the  granules 
are  green,  greenish  yellow,  brown,  or  black.  The  green  and  vellow  ones 
are  trans}3arent,  while  the  brown  and  black  are  nearl}"  or  (piite  opacjue 
Under  crossed  nicols  the  granules  are  either  entirely  dark  or  show  a  very 
faint  lightening,  hardly  sufiicient  to  disclose  a  color.  Here  and  there 
incipient  alterations  to  chert,  griinerite,  cummingtonite,  ov  actinolite, 
scarcely  discernible  in  ordinary  light,  give  low  ])olarization  colors  in 
minute  s|)ots  and  make  the  term  aggregate  ])olarization  applicalde.  In 
ret)ecte<l   light   the  transparent  green  and  vellow  granules  appear  black  or 


PLATE  VIII. 


103 


PLATE   VIII. 

GREENALITE    KOCK.  I 

Fig.  a. — Greenalite  rock.  Specimen  4.5647.  From  near  Dulnth,  Missabe  and  Northern  Raihvay 
track,  1  mile  south  of  Virginia.  Granules  of  greenalite,  but  little  altered,  stand  in  a  matrix  of  chert. 
Described  pp.  101-115. 

Fig.  xV. — Portion  of  surface  of  specimen  shown  in  ,1  slightly  magnified  to  show  greenalite  granules 
to  better  advantage. 

Fig.  B. — Interbanded  greenalite  and  slate  rock.  Specimen  45176.  From  100  paces  north  500 
paces  west  of  SE.  corner  of  sec.  22,  T.  59  N.,  R.  15  W.  Natural  size.  The  black  portion  of  the  rock 
is  slate  and  the  green  portion  is  made  up  of  greenalite  granules  lying  in  a  matrix  of  chert.  Greenalite 
is  characteristically  associated  with  slaty  layers  in  the  iron  formation:  indeed,  it  is  due  to  their 
protection  that  greenalite  has  been  retained  in  comparatively  unaltered  form.     Described  pp.  101-115. 

104 


U.  S.  GEOLOGICAL   SURVEY 


MONOGRAPH    XLMI    PL.   Vlll 


GREENALITE  ROCK. 


PLATE   IX. 


105 


PLATE  IX. 

PHOTOMICROGRAPHS  OF  GREENALITE  GRANULES. 

Fig.  a. — Greenalite  rock.  Specimen  45178,  slide  15652.  From  100  paces  north  500  paces  west 
of  the  southeast  corner  of  sec.  22,  T.  59  K.,  R.  15  W.  Without  analyzer,  x  50.  The  slide  is  selected 
to  show  both  the  fresh  and  slighth'  altered  giunules.  Note  the  peculiar  greenish-yellow  color  of  the 
granules,  their  irregular  shape,  and  their  curving  tails,  which  seem  in  some  cases  to  connect  with 
adjacent  granules.  The  homogeneous  greenish-}'ellow  colors  represent  the  unaltered  parts.  The 
bright-green  and  dark-green  colors  represent  griinerite  which  has  been  developed  from  the  alteration 
of  the  greenalite.     The  dark  green  is  perhaps  in  small  part  iron  oxide.     Described  pp.  101-115. 

Fig.  B. — The  same  with  analyzer,  x  50.     The  unaltered  portions  of  the  granules  are  nearly  or 
quite  dark  under  crossed  nicols.     Where  the  granules  have  altered  ti  griinerite  the  polarization  colors 
appear.     The  matrix  consists  of  fine-grained  chert  in  which  the  individual  particles  are  very  irregular 
in  shnpc  and  size.     Described  i)p.  101-115. 
106 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH    XLIII    PL.    IX 


PHOTOMICROGRAPHS  OF  GREENALITE  GRANULES 


THE  BIWABIK  FORMATION.  107 

dark  green  or  dark  yellow,  while  the  opaque  brown  and  black  granules 
exhibit  a  rough  light-green  surface.  Were  it  not  for  the  light-green  surface 
in  reflected  light  certain  of  the  opaque  dark-brown  granules  would  be  mis- 
taken for  iron  oxide  in  ordinarj-  and  polarized  light. 

The  matrix  of  the  rocks  containing  the  unaltered  green  granules  varies 
widely  in  amount,  from  a  mere  interstitial  filling  to  an  abundant  mass  in 
which  the  granules  are  widely  separated.  The  matrix  may  be  almost  pure 
chert;  it  may  be  nonalumiuous,  monoclinic  amphibole,  actinolite,  griinerite, 
or  cummingtonite ;  it  may  be  largely  iron  or  calcium  carbonate,  although 
where  the  carbonate  is  abundant  the  granules  are  usually  sparce"and  irreg- 
ular; it  may  consist  of  any  combination  of  chert,  amphibole,  aiid  carbonate, 
with  a  small  amount  of  accessory  iron  oxide. 

Originally  the  matrix  may  have  had  a  somewhat  different  character. 
In  the  rocks  containing  the  least  altered  granules  the  matrix  is  predom- 
inantly chert  and  subordinately  light-colored  amphiboles  and  carbonate. 
As  the  rocks  become  altered  they  contain  more  iron  oxide  and  dark  amphi- 
boles, which  will  be  shown  on  a  subsequent  page  to  develop  from  the 
alteration  of  the  granules.  The  lighter  amphiboles  are  themselves  known 
to  be  a  secondary  development  from  chert  and  carbonate  rocks.  It  seems 
likely,  therefore,  that  the  original  matrix  of  the  green  granules  was  largely 
chert  and  in  small  part  carbonate.  In  the  freshest  rocks  now  found  the 
chert  is  much  recrystallized  and  the  original  carbonate  is  largely  leached 
out  or  replaced  by  actinolite. 

The  specific  gravity  of  the  unaltered  granules  can  not  be  satisfac- 
torily determined,  because  of  the  practical  impossibility  of  separating'  the 
granules  from  the  matrix.  Determinations  of  the  specific  gravity  of  the 
rock  as  a  whole  give  results  ranging  from  2.7  to  3.  As  the  matrix  is  largely 
quartz  in  the  form  of  chert,  which  is  known  to  have  a  specific  gravity  in 
the  neighborhood  of  2.65,  the  figures  above  gi^^en  for  the  unaltered  rock 
are  too  low  for  the  granules  themselves,  although  their  incijiient  alterations 
to  iron  oxide  and  amphiboles  tend  to  raise  the  specific  gravity.  So  far  as 
the  matrix  is  colorless  amphibole,  it  is  apparent  that  the  specific  gravity  of 
the  green  granules  is  lower  than  the  fig'ures  obtained  for  the  rock,  for  the 
specific  gravity  of  the  colorless  amphiboles  is  above  3.  One  exceptionally 
fresh  specimen  in  which  the  granules  lie  in  a  matrix  of  chert  gave  a  result 
of  2.7.     The  matrix  in  this  case  n:iakes  up  somethiug  more  than  half  of  the 


108 


THE  MESABI  IRON-BEARING  DISTRICT. 


rock  mass,  and  it  therefore  seems  pi-obable  that  the  true  specific  gravity  of 
the  granules  is  a  Httle  above  2.75. 

Four  analys-es  of  rocks  containing  the  least  altered  granules  observed 
have  been  made  by  Mr.  George  Steiger  of  the  United  States  Geological 
Survev.  He  found  that  by  treatment  with  hot  concentrated  hydrochloric 
acid  most  of  the  granules  and  their  associated  alteration  products  dissolved 
out,  leaving  a  residue  of  almost  clear  silica,  which  probably  mainly 
represents  the  matrix. 

Analyses  of  greenaUte  rocks. 


1. 

2 

3 

4. 

sol. 

Insol. 

Sol. 

Insol. 

Sol. 

Insol. 

SiO,                    

13.45 

.37 

15.00 

10.28 

2.33 

.28 

None. 

None. 

2.50 

4.17 

None. 

2.04 

None. 

48. 45 
.64 



• 

«19.30 

.61 

13.83 

17.  57 

3.22 

None. 

None. 

None. 

2.38 

5.74 

None. 

None. 

None. 

36.50 

.76 



33.11 
.56 

6.44 
30.93 

5.  35 
None. 
None. 
None. 

1.34 

6.13 
None. 
None. 
None. 

13.01 
2.60 


650.96 

ALOa      

1.09 

Fe,0, 

5.01 

FeO 

30.37 

:MgO 

5.26 

CaO 

.04 

Na.,0 

None. 

K.,0 

None. 

HjO—                             .     .            -.     .. 

.  75 

H,0-      

6.41 

TiO, 

None. 

CO., 

None. 

P.O. 

None. 

S 

Trace. 

MnO                      

None. 

BaO            

None. 

r'nrhfin  in  nrp^nip  rnat^^T 

.21 

Undetermined 

..52 

.15 

.38 

50.42 
49.61 

49.61 

62.65 
37.41 

37.41 

83.86 
15.99 

15.19 

100. 10 

100. 03 

100.06 

99.85 

1 

"Of  which  3.3  was  found  in  the  rock  upon  treatment  with  HCl.  (probably  opal). 
6  Of  which  23.96  is  soluble. 

1.  Specimen  45758.  From  250  paces  west,  83  paces  north,  of  the  we.st  quarter  popt,  fee.  35,  T.  59 
N.,  R.  15  W.  The  finely  ground  rock  was  evaporated  on  the  water  bath  to  dryness,  with  50  cc.  of  1-1 
nCl,  taken  np  witli  water  slightly  acidified  with  Ilt'l,  and  filtered.  Soluble  silica  was  then  determineil 
in  this  residue  by  boiling  with  5  per  cent  solution  of  Na.X^O,,.  A  determination  nf  soluble  SiO.j  wa.s 
then  made  in  the  rock  before  treatment  with  HCl  and  subtracted  from  the  first  soluble  SiO,  found, 
which  gave  the  figure  for  SiOj  in  the  soluble  portion. 


THE  BIWABIK  FORMATION.  109 

2.  Specimen  45765.  Prom  test  pit  in  Cincinnati  mine.  The  soluble  portion  was  found  by  evaporat- ' 
ing  to  dryness  on  the  water  bath  with  50  cc.  of  1-1  HCl,  and  taking  up  with  water  slightly  acidified 
with  HCl.  The  residue  was  then  boiled  fifteen  minutes  with  a  5  per  cent  solution  of  NaX'Oj  to  dis- 
solve any  soluble  silica,  this  silica  determined  and  placed  with  the  soluble  portion.  The  residue 
was  ignited  and  finally  heated  for  fifteen  minutes  over  the  blast  lamp,  weighed,  and  then  a  rough 
analysis  made,  which  is  found  in  the  second  column.  The  small  amount  of  iron  shown  in  the 
insoluble  portion  could  easily  have  been  carried  down  mechanically.  A  determination  of  soluble 
silica  was  then  made  in  the  rock  before  treatment  with  HCl  and  found  to  be  3.  .3  per  cent.  Subtracting 
this  froui  the  total  soluble  silica  16  per  cent  of  soluble  silica  remains  for  the  part  dissolved  in  HCl. 

3.  Specimen  45766.  From  test  pit  in  Cincinnati  mine.  The  finely  ground  rock  was  evaporated 
on  the  water  bath  to  dryness,  with  50  cc.  of  1-1  HCl,  taken  up  with  water  slightly  acidified  with  HCl, 
and  filtered.  Soluble  silica  was  then  determined  in  this  residue  liy  boiling  with  5  per  cent  solution 
of  jSTajCOj.  A  determination  of  soluble  SiO,  was  then  made  in  the  rock  before  treatment  with 
HCl  and  subtracted  from  the  first  soluble  SiO.j  found,  which  gave  the  figure  for  SiO^  in  the  soluble 
portion. 

4.  Specimen  45180.  From  500  paces  west,  100  paces  north  of  the  southeast  corner  of  sec.  22,  T.  59 
N.,  R.  15  W.     Owing  to  presence  of  organic  matter  the  determination  of  ferrous  iron  is  probably  high. 

The  intei'pretation  of  these  results  requires  separate  discussion  because 
of  the  variation  in  nature  and  amount  of  associated  minerals. 

1.  Green  and  brown  transparent  granules,  and  opaque  brown  and 
black  ones,  containing  small  amounts  of  secondary  chert,  carbonate,  and 
limonite,  stand  in  a  matrix  of  chert.  One  large  pebble-like  area  consists  of 
granular  limonitic  material  with  a  small  amount  of  carbonate.  In  this  area 
the  outlines  of  granules  can  be  distinctly  seen,  and  the  limonite  cleai-ly 
results  from  alteration  of  the  granules. 

The  undissolved  portion  probably  mainly  represents  the  matrix. 
The  dissolved  portion  probably  mainly  represents  the  green  granules, 
the  limonite',  and  carbonate.  In  calculating  the  composition  of  the  green 
granules,  the  carbon  dioxide,  with  enough  of  the  bases  to  satisfy  its  valence, 
may  be  eliminated.  As  the  microscope  does  not  show  conclusively 
whether  carbonate  is  calcite,  dolomite,  or  siderite  in  the  calculation,  the 
bases  may  be  supposed  to  be  combined  with  carbon  dioxide  in  proportion 
to  their  strength.  Thus  the  calcium  oxide  jDresent,  0.28  per  cent,  may  be 
supposed  to  be  combined  with  carbon  dioxide,  which  would  leave  1.82  per 
cent  carbon  dioxide  available  for  combination  with  other  bases.  Magnesium 
oxide  is  next  in  strength,  and  1.67  per  cent  would  be  required  to  combine 
with  the  remaining  carbon  dioxide.  This  would  leave  0.66  per  cent  of 
magnesium  oxide,  which  may  be  sujDposed  to  belong  wirh  the  green  granules 
or  with  the  associated  alteration  jDroducts.  It  is  possible  that  the  carbon 
dioxide  may  be  combined  in  part  with  ferrous  iron,  but  in  the  absence  of 
definite  information  the  above  combination  is  supposed  to  hold.     Whatever 


110  THE  MESABI  IRON-BEARING  DISTRICT. 

the  combination,  it  will  be  noted  that  the  total  amount  of  carbon  dioxide  is 
so  small  that  the  exact  determination  of  the  combination  is  not  a  matter  of 
consequence.  After  making  deductions  for  the  carbonates,  the  composition 
of  the  green  granules  and  the  associated  alteration  products  is  as  follows: 

SiO;, - 13.45 

AlA - - 37 

FeA 15.00 

FeO 10.28 

MgO 66 

H^O  above  110° 4-17 

We  know  that  iron  protoxide  and  magnesium  oxide  never  occur  in 
rocks  except  in  combined  form.  The  magnesium  compound  known  as 
brucite  (Mg(0H)2)  has  not  been  noted  in  these  rocks.  It  is  necessary  to 
assume  that  sufficient  soluble  silica  is  combined  with  ferrous  iron  and 
magnesia  to  satisfy  their  valence,  and  when  this  amount  is  deducted  little 
or  none  is  left  for  combination  with  the  ferric  iron.  It  is  thus  clear  that  a 
large  portion  of  the'  green  granules  is  ferrous  silicate.  It  is  further  clear 
that  the  ferric  iron  shown  by  the  analyses  is  in  the  form  of  ferric  oxide, 
and  thus  probably  secondary.  If  original,  it  may  still  be  independent 
of  the  green  granules,  and  there  remains  only  a  possibility  that  the  ferric 
oxide  may  be  an  original  constituent  of  the  green  granules  themselves. 
This  is  in  accord  with  the  microscopic  observation  of  the  presence  of  a 
considerable  amount  of  limonite  in  the  slide.  The  limonite  is  secondary 
and  independent  of  the  green  granules,  and  thus  tlie  ferric  iron,  with 
the  water  combined  with  it,  may  be  eliminated  from  the  discussion  of  the 
composition  of  the  green  granules.  Of  the  combined  water  shown  in  the 
analysis  2.53  per  cent  would  be  required  for  combination  with  the  ferric 
iron,  on  the  assumption  that  the  latter  is  all  in  the  form  of  limonite,  thus 
leaving  1.64  per  cent  of  water  probably  belonging  with  the  green  granules. 
It  is  concluded,  therefore,  that  the  material  of  the  green  granules  is  essen- 
tially a  livdrated  feiTous  silicate,  with  a  small  amount  of  magnesium  and 
possibly  a  slight  amount  of  ferric  oxide  and  alumina.  The  total  absence 
of  the  alkalies  and  phosphorous  is  to  be  noted. 

2.  In  this  roclv  the  granules  are  green  and  transparent  and  in  part 
dark  brown,  Ijlack,  and  opaque,  and  show  a  very  slight  and  practically 
negligible  alteration  to  greenish  and  colorless  amphibole.  The  matrix  is 
chert.  The  undissolved  portion  probably  mainly  repi'esents  the  matrix. 
The  dissolved  portion  mainly  represents  tlie  green  granules. 


THE  BIWABIK  FOEMATION.  Ill 

The  composition  of  the  green  granules,  tog'ether  with  their  minute 
alteration  products,  is  therefore  as  follows: 

SiO^ 16. 00 

Al.Oj 61 

Fe^Oj 13.  83 

FeO 17. 57 

MgO 3.22 

HaO  above  110° 5.  74 

Arguing  as  above,  we  know  that  iron  protoxide  and  magnesium  oxide 
never  occur  in  rocks  except  in  combined  form.  The  magnesium  compound 
known  as  brucite  (Mg(0H)2)  has  not  been  noted  in  these  rocks.  It  is 
necessary  to  assume  that  sufficient  soluble  silica  is  combined  with  ferrous 
iron  and  magnesium  oxide  to  satisfy  their  valence,  and  when  this  amount  is 
deducted  little  or  none  is  left  for  combination  with  the  ferric  iron.  From 
this  it  is  clear  that  a  large  portion  of  the  green  granules  is  ferrous  silicate. 
It  is  further  clear  that  the  ferric  iron  shown  by  the  analyses  is  in  the  form 
of  ferric  oxide,  and  thus  probably  secondary.  If  original  it  may  still  be 
independent  of  the  green  granules,  and  there  remains  only  a  possibility 
that  the  ferric  oxide  may  be  an  original  constituent  of  the  green  granules 
themselves,  but  as  no  iron  oxide  can  be  certainly  observed  in  the  slide  this 
possibility  must  be  recognized.  Thus  most  of  the  ferric  oxide,  together 
with  any  water  which  may  be  combined  with  it,  may  be  eliminated  from 
the  discussion  of  the  composition  of  the  green  granules.  If  the  iron  oxide 
were  all  limonite  it  would  not  reqiiire  all  the  combined  water,  and  thus  a 
considerable  portion  of  the  combined  water  must  belong  with  the  green 
granules.  Thus  the  material  of  the  green  granules  appears  to  be  mainly 
hydrated  ferrous  silicate  with  a  small  amount  of  magnesium  and  perhaps 
also  small  portions  of  ferric  oxide  and  alumina  The  entire  absence  of  the 
alkalies  and  phosphorus  is  to  be  noted. 

3.  The  granules,  of  a  greenish- yellow  color,  are  slightly  altered  to 
colorless  and  in  part  shghtly  greenish  amphibole,  and  lie  in  a  matrix  of 
colorless  and  shghtly  greenish  amphibole,  associated  with  a  subordinate 
amount  of  chert.  There  is  present  in  addition  a  small  amount  of  oxide  of 
iron  which  may  be  either  more  or  less  hydrated  hematite  or  magnetite. 
The  insoluble  portion,  which  is  small  in  amount,  is  shown  by  the  analysis 
to  be  mainly  sihca,  with  a  subordinate  amount  of  ferric  iron.  The  soluble 
portion  contains  the  greenish  granules,  the  amphiboles,  and  the  major 
portion  of  the  iron  oxide      As  there  is  microscopic  evidence  both  in  this 


112  THE  MESABI  IRON-BEARING  DISTRICT. 

rock  and  in  the  iron-foriiiaticm  rocks  as  a  whole  tliat  the  araphibf)les  resuh 
from  the  alteration  of  the  green  granules,  and  in  many  cases, at  least  merely 
by  recrystallization  and  dehydi-ation  of  the  substance  of  the  green  granules, 
it  is  apparent  that  no  great  error  will  be  introduced  if  the  sul)stance  of  the 
amphiboles  be  considered  together  with  the  rest  of  the  soluble  material  in 
determining  the  approximate  composition  of  the  green  granules.  The 
composition  of  the  green  granules,  together  with  amphiboles  resulting  from 
their  alteration,  and  the  black  iron  oxide,  is: 

SiOs 33.11 

AI263  - 56 

Fe^Oa 6.  44 

FeO - : - - 30. 93 

MgO - - 5.  35 

HoO  above  110° 6. 13 

The  percentage  of  ferric  oxide  shown  is  small,  and  it  is  thought  that  it 
is  largely  accounted  for  by  the  oxide  seen  in  the  slide  and  thus  ought  not 
to  be  counted  as  belonging  with  the  green  granules.  If  a  portion  of  the 
iron  is  magnetite,  then  a  small  percentage  of  ferrous  iron  belongs  to  it. 
If  it  were  all  magnetite,  about  3  per  cent  of  the  ferrous  iron  would  be  so 
combined,  and  hence  the  true  figure  is  probably  less  than  this.  At  most 
but  a  very  small  percentage  of  the  combined  water  can  be  supposed  to 
belong  with  the  ferric  oxide,  and  also  but  little  can  belong  with  the 
amphiboles;  the  large  percentage  of  combined  water  shown  by  the  analyses 
belongs  largely  to  the  substance  of  the  green  granules.  The  analysis 
therefore  shows  the  original  green  material  to  be  essentially  a  hydrous 
ferrous  silicate  with  a  considerable  percentage  of  magnesium,  and  perhaps 
small  amounts  of  ferric  oxide  and  alumina.  The  entire  absence  of  the 
alkalies  and  phosphorus  is  to  be  noted. 

4.  The  granules  are  in  pai-t  yellowish  brown  and  transparent,  and  in 
part  dark  brown,  black,  and  opaque,  the  latter  showing  the  characteristic 
rousrh,  ofreen  surface  in  reflected  light.  They  are  largely  fresh,  but  a 
number  of  them  show  slight  alterations  to  colorless,  light-brown,  and  light- 
green  amphibole.  There  is  present  also  a  small  amount  of  black  iron  oxide. 
The  matrix  is  mainly  a  felted  mass  of  colorless  amphibole,  with  a  slightly 
greenish  pleochroism,  with  high  double  refraction  and  low  angle  of  extinc- 
tion, which  corresponds  in  its  properties  to  actinolite.  The  amphibole 
within  and  adjacent  to  some  of  the  granules  may  be  seen  in  all  stages  of 


THE  BIWABIK  FORMATION.  113 

development  through  the  alteration  of  the  granules,  and  it  is  probable  that 
all  of  the  amphibole  has  developed  in  this  way. 

In  this  analysis  the  composition  of  the  entire  rock  was  first  determined 
and  then  the  soluble  silica  found.  No  determination  was  made  of  the 
substance  left  in  the  residuum  after  the  treatment  with  hydrochloric  acid, 
and  it  is  not  possible  to  state  how  the  amphibole  in  the  mati'ix  acted  under 
the  treatment.  The  analysis,  therefore,  affords  no  direct  evidence  of  the 
composition  of  the  green  granules.  But  it  seems  probable  that  at  least 
a  part  of  the  amphibole  went  into  solution  with  the  hydrochloric-acid 
treatment.  The  soluble  portion  would,  then,  contain  the  original  green 
material,  an  unknown  quantity  of  amphibole,  and  a  slight  amount  of  black 
oxide  of  iron.  If  the  amphibole  which  may  have  gone  into  solution  be 
considered  as  an  alteration  of  the  original  green  material  essentially  by 
simple  recrystallization,  as  it  certainly  is  in  the  iron  formation  as  a  whole, 
then  its  ingredients  need  not  be  separately  considered  in  arriving  at  an 
approximation  of  the  composition  of  the  original  green  material.  Most 
of  the  ferric  iron  shown  in  the  analysis  is  accounted  for  by  the  black  oxide 
ol  iron  seen  in  the  slide.  As  the  black  oxide  of  iron  is  at  least  partly 
magnetite  a  small  percentage  of  ferrous  iron  must  be  supposed  to  belong 
with  it.  If  the  ferric  oxide  all  belongs  to  magnetite,  2.2  per  cent  would  be 
so  required;  so  the  ti'ue  figure  is  probably  something  less  than  this.  A 
still  further  deduction  must  be  made  from  the  ferrous  iron,  as  the  analyst 
makes  the  statement  that  the  percentage  of  ferrous  iron  is  probably  high 
because  of  the  presence  of  organic  matter.  Only  a  very  small  percentage 
of  the  combined  water  can  be  accounted  for  by  combination  with  the  oxide 
of  iron,  for  this  is  in  small  quantity,  and,  moreover,  largely  magnetite. 
Neither  can  any  of  the  water  be  supposed  to  belong  with  the  amphiboles, 
for  the  latter  are  nearly  anhydrous.  Thus  most  of  the  water  belongs  with 
the  substance  of  the  green  granules.  It  is  clear  that  at  least  a  part  of  the 
material  of  the  dissolved  portion  is  a  hydrous  ferrous  silicate,  and  it  is 
certain  that  this  part  belongs  with  the  unaltered  green  granules. 

Assembling  the  above  results,  it  appears  that  the  ferric  iron  occurs  in 
the  rock  mainly  as  sesquioxide,  for  the  soluble  sihca  is  accounted  for  by 
the  ferrous  iron  and  magnesia  present,  leaving  none  for  the  ferric  iron;  that 
in  three  slides  of  the  four  of  the  rocks  analyzed  the  ferric  oxide  may  be 
observed  to  be  pi'esent  and  to   be  probably  secondary;    and,  hence,  that 

MON  XLIII — 03 8 


114 


THE  MESABI  IRON-BEARING  DISTRICT. 


the  iron  oxide  shown  by  the  analyses  is  mainly  secondary  and  not  to  be 
considered  as  belonging  with  the  substance  of  the  unaltered  granules.  It 
appears  further  that  the  akimina  and  lime  are  in  such  small  quantity  as  to 
be  practically  negligible.  It  appears  still  further  that  there  is  far  more 
than  enouffh  combined  water  to  combine  with  the  ferric  iron  to  form  ferric 
hydrate,  and  thus  that  a  considei'able  portion  of  combined  water  shown 
by  the  analyses  may  be  taken  to  belong  to  the  green  granules.  Finally, 
it  appears  that  the  substances  which  can  not  be  accounted  for  in  any 
other  way  and  which  clearly  belong  with  the  green  granules  are  silica, 
ferrous  iron,  magnesium  oxide  in  small  proportions,  and  water.  It  is  there- 
fore concluded  that  the  substance  of  the  green  granules  is  essentially  a 
hydrous  ferrous  silicate  with  a  subordinate  amount  of  magnesium,  and 
that  if  ferric  iron  is  present  at  all  as  an  original  constituent  of  the  green 
granules  it  is  in  small  quantity. 

This  conclusion  is  essentially  in  accord  with  that  reached  by  Dr.  J.  E. 
Spurr  in  his  report  on  the  Mesabi  district  published  in  1894." 

Having  concluded  the  substance  of  the  green  granules  to  be  mainly 
silica,  ferrous  iron,  magnesium  oxide,  and  water,  we  may  ascertain  whether 
or  not  there  is  any  uniformity  in  the  proportions  of  these  elements.  The 
ratios  of  the  silica,  ferrous  iron,  and  magnesium  in  the  four  analyses, 
calculated  on  the  basis  of  100,  appear  in  the  subjoined  table.  The 
percentage  of  water  is  not  included  for  the  obvious  reason  that,  while  it  is 
certain  that  much  of  it  belongs  with  the  granules,  no  quantitative  estimate 
can  be  made  of  its  amount  because  of  the  uncertainty  as  to  the  portion 
which  belongs  with  the  ferric  hydrate. 


1. 

2. 

3. 

4. 

Average. 

SiOj                                                       -  - 

55.1 

42.1 

2.8 

43.7 

47.5 

8.8 

47.7 

44.6 

7.8 

40.2 

50.9 

8.9 

46.8 

FeO                                .             

46.3 

MgO 

7.1 

Tlie  relative  proportion  of  the  ferrous  ii'on  and  silica  above  shown 
suggests  a  combination  of  the  two  on  the  basis  of  one  molecule  of  each. 
Theoretically  the  percentages  of  the  two  in  such  a  combination  would  be — 

Silica 45. 62 

Ferrou.s  iron 54. 38 


aGeol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  10. 


THE  BIWABIK  FORMATION.  115 

The  average  of  the  ferrous  iron,  46.3,  is  about  8  per  cent  less  than 
the  theoretical  percentage.  The  magnesium  oxide,  which  has  a  higher 
combining  power  than  the  iron,  more  than  makes  up  for  this  deficiency. 

On  a  subsequent  page  (pp.  141-143)  is  given  an  analysis  of  a  rock  in 
which  the  green  granules  have  been  altered  to  a  dark-green  and  brown 
amphibole,  probably  griinerite,  apparently  through  simple  recrystallization 
and  dehydration.  The  alteration  has  occurred  under  deep-seated  conditions, 
and  it  is  probable  that  little,  if  any,  addition  or  subtraction  of  material  has 
taken  place,  other  than  that  involved  in  dehydration."  The  composition 
of  the  amphibole  ought  to  give  a  clew  to  the  composition  of  the  original 
green  substance.  It  is  there  found  that  the  principal  constituents  of  the 
amphibole   are   silica  and  fen-ous  iron,  in  the  following  proportions: 

SiOj 47.5 

FeO 52.5 

The  correspondence  of  these  percentages  with  those  above  given  is 
evident. 

It  is  apparent  that  the  above  results  are  not  sufiiciently  accordant  to 
show  that  the  substance  under  discussion  has  a  definite  and  uniform  compo- 
sition. On  the  other  hand,  the  impurities  and  alterations  cause  such  vari- 
ations that  it  can  not  be  said  that  the  green  granules  do  not  have  definite 
chemical  composition.  If  the  granules  do  have  a  definite  composition,  the 
above  results  indicate  the  most  probable  formula  to  be  Fe(Mg)OSi02nH20. 

Dr.  Sjjurr,  after  his  study  of  the  green  granules,  concluded  to  call 
them  "  glauconite."  In  view  of  the  fact  that  potash  is  insisted  upon  as  one 
of  the  essential  constituents  of  glauconite  by  most  mineralogists  (see  pp. 
239-243),  the  entire  absence  of  potash  in  the  substance  under  discussion  is 
taken  to  preclude  the  application  of  the  term  glauconite.  The  substance 
apjjarently  corresponds  to  no  known  mineral  species.  As  it  will  be  neces- 
sary and  convenient  to  have  a  term  by  which  to  refer  to  it  in  the  present 
discussion,  the  name  "greenalite"  is  coined  for  use  in  this  report. 

The  origin  of  greenalite  and  the  details  of  the  similarities  and  differ- 
ences between  greenalite  granules  and  granules  of  glauconite,  concretions 
of  iron  oxide  and  chert,  and  other  granule  and  concretionary  structures, 
are  discussed  in  Chapter  IX,  on  the  origin  of  the  iron  ores. 

oin  a  monograph  on  Metamorphism  (in  press)  C.  E.  Van  Hise  emphasizes  the  fact  that  alter- 
ations in  tne  deep-seated  "zone  of  anamorphism"  for  the  most  part  involve  no  considerable  transfers 
of  material. 


116  THE  MESABI  IRON-BEARING  DISTRICT. 

FERKUGINOUS,  AMFHIBOLITIC,  SIDERITIC,  AND    CALCAREOUS    CHERTS. 

The  followiug  description  applies  to  the  normal  types  of  chert  occur- 
ring through  the  ceiitral  and  western  portions  of  the  range.  The  highly 
metamorphosed  chert  characteristic  of  the  east  end  of  the  range  is  given  a 
separate  description  on  a  subsequent  page. 

The  cherts  are  gray,  yellow,  red,  brown,  or  green  rocks,  with  irregular 
bands  and  shots  and  granules  of  iron  oxide,  varying  in  quantity  from 
predominance  almost  to  disappearance  (PI.  X-XII).  A  slight  brecciation 
thoroughly  recemented  maj^  be  occasionally  observed,  and  a  pitted 
surface,  due  to  the  solution  of  certain  of  the  constituents,  is  not  uncommon. 
The  iron  oxide  is  riaainly  intermediate  between  hematite  and  limonite,  and 
to  a  subordinate  extent  is  magnetite,  and  its  color  accordingly  ranges  from 
red  to  yellow  or  to  black.  The  variety  of  colors  of  the  chert  and  the  iron 
oxide,  their  irregular  association,  and  their  variation  in  relative  abundance 
giA'e  the  cherts  most  highly  varied  aspects;  ^^et  no  phase  of  the  cherts 
is  likely  to  be  mistaken  for  any  other  rock  by  anyone  reasonably  familiar 
with  the  iron-formation  rocks  of  the  Lake  Superior  region.  To  the  casual 
observer,  the  massive,  lighter-colored  cherts,  containing  little  iron  oxide, 
resemble  quai'tzite,  and,  indeed,  have  been  frequently  so  called.  However, 
the  splintery  fracture  of  the  chert  and  the  absolute  lack  of  rounded  clastic 
grains,  aside  from  the  usual  content  of  iron  oxide  in  layers  or  spots  or 
minute  grains,  are  unfailing  criteria  for  the  discrimination  of  the  two.  The 
feiTuginous  chei'ts  differ  from  the  jaspers  or  jaspilites  of  the  old  ranges  of 
Lake  Superior  in  lacking  their  even  banding  and  brilliant  red  color  as 
well  as  the  microscopic  features  described  below. 

When  studied  under  the  microscope,  it  is  apparent  that  all  the  rocks 
here  described  as  chert  are  genetically  connected.  In  looking  over  250 
slides  but  few  have  been  observed  which  do  not  show  some  evidence  of  the 
derivation  of  the  rock  from  the  greenalite  rocks  above  described.  The 
granule  shapes  are  still  largely  preserved,"  but  the  alterations  liave  tended 
in  some  cases  to  make  the  sliapes  more  irregular  and  to  partly  (U-  \\holly 
obliterate  them.  The  alteration  of  the  granules  has  been  almost  entirely 
metasomatic,  for  there  is  little  evidence  of  dynamic  movement  resulting  in 
the  breaking  up  of  the  constituents  of  the  rock. 

rtRpurr  has  applicMl  to  tliiH  tfxtiire  (lie  tvnu  "HpiiUi'd  granular."  Gcul.  Nat.  Hist.  Survey 
Minnesota,  lUiU.  No.  lU. 


THE  BIWABIK  FORMATION.  117 

The  greenalite  has  been  replaced  by  cherty  quartz,  magnetite,  hematite, 
hmouite,  siderite,  calcite,  griinerite,  cummingtonite,  actinoHte,  epidote- 
zoisite,  or  any  combination  of  them.  The  extent  and  nature  of  the 
alteration  replacement  vary  within  wide  limits.  The  granule  may  be 
mainly  greenalite,  showing  incipient  crystallization  of  quartz,  griinerite, 
or  actinolite,  visible  onl}^  under  crossed  nicols.  The  granules  may  be 
represented  almost  wholly  by  hematite,  limonite,  magnetite,  intermediate 
varieties,  or  any  combination  of  them.  The  oxides  may  be  arranged 
iiTegularly  or  concentrically.  In  the  iron  ores  the  granules  are  entirely 
represented  by  iron  oxide,  although  their  shapes  are  in  part  obliterated. 
The  granule  may  be  represented  ahnost  wholly  by  chert,  which  may 
be  distinguished  from  that  of  the  matrix  by  its  coarser  or  finer  texture,  or, 
if  not  by  texture,  by  distribution  of  pigment.  In  ordinary  light  chert 
granules  may  be  marked  by  the  pigments  which  in  parallel  polarized  light 
are  completely  obscured  by  the  crystallization  of  the  chert  (PL  XV, 
figs.  C  and  D),  or  the  granules  may  not  be  seen  in  ordinary  light  and  be 
conspicuous  imder  crossed  nicols  because  of  the  crystallization  (specimen 
45191).  Or  the  crystallization  of  the  chert  may  have  entirely  obliterated 
the  granules  for  much  of  the  slide,  both  in  ordinary  and  polarized  light. 
The  granules  may  be  represented  entirely  by  green,  yellow,  and  brown 
griinerite,  cummingtonite,  or,  perhaps,  actinolite,  or  all  (specimen  45497), 
which  in  ordinary  light  may  be  scarcely  distinguishable  from  the  unaltered 
greenalite  granules,  but  which  become  apparent  under  crossed  nicols  by 
their  double  refraction.  The  granule  may  be  represented  by  calcite  or 
siderite  in  rhombs  or  irregular  grains,  sometimes  showing  zonal  growth, 
which,  for  the  most  part,  are  clearly  replacements  of  the  granules  (speci- 
mens 45171,  45172,  45174,  45219,  45222).  Most  commonly  the  granules 
are  represented  by  a  combination  of  any  or  all  of  the  minerals  above 
named.  Of  these  combinations,  that  of  chert  and  iron  oxide  stands  first. 
The  two  substances  occur  in  all  proportions  with  a  great  variety  of  arrange- 
ment. The  two  may  be  irregularly  intermingled,  or  the  iron  oxide  may 
form  a  rim  about  a  cherty  interior,  or,  though  not  frequently,  the  chert 
and  iron  oxide  may  be  in  concentric  layers  in  the  manner  of  normal  con- 
cretions, or  polygonal  areas  of  fine  chert  may  contain  spots  of  iron  oxide  in 
the  center  of  each,  as  well  as  a  rim  of  iron  about  their  peripheries  (PI.  XIV, 
figs.   A    and   B),  suggesting    an    organic    structure.     The    alteration    and 


118  THE  MESABI  IRON-BEARING  DISTRICT. 

replacement  of  the  greenalite  and  the  conditions  favoring  the  development 
of  the  different  minerals  are  discussed  under  the  origin  of  the  ores. 

In  addition  to  the  derivatives  of  the  greenalite  granules  there  are 
present  a  few  concentric  concretions  of  iron  oxide  and  chert  about  quartz 
(see  PI.  XIII,  fig.  D),  which  may  have  been  secondarily  developed 
from  some  substance  other  than  the  greenalite.  These  are  similar  to  con- 
cretions in  the  Penokee-Gogebic  iron-bearing  formation, "  where  they  have 
developed  from  the  alteration  of  an  iron  carbonate.  (See  PI.  XVI,  fig.  A.) 
The  secondary  concretions  in  the  Mesabi  district  may  also  be  developments 
from  iron  carbonates,  which  are  now  associated  with  unaltered  portions  of 
the  formation  and  probably  existed  formerly  in  the  portions  which  are  at 
present  altered.  The  secondary  concretions  are  different  from  the  greena- 
lite granules  in  their  beautifully  developed  concentric  structure,  as  may 
be  noted  by  a  comparison  of  the  figures  of  PI.  XIII.  While  a  few  of 
the  granules  themselves  have  a  concentric  structure  resulting  from  zonal 
alteration,  this  is  usually  poorly  developed  and  there  is  ordinarily  little 
difficulty  in  distinguishing  it  from  that  of  the  secondary  concretion,  though 
in  some  cases  it  is  possible  that  some  of  the  supposed  secondary  concretions 
formed  from  carbonate  may  be  really  secondary  alterations  of  original 
graimles. 

Spherulites  of  epidote,  rarely  to  be  observed,  while  in  part  replacements 
of  the  granules,  are  also  clearly  secondary  developments  in  the  matrix. 

The  occurrence  of  true  secondary  concretions  associated  with  the 
derivatives  of  greenalite  granules  in  the  Mesabi  district  suggests  that  greena- 
lite granules  may  be  present  with  the  secondar}^  concretions  in  the  Gogebic 
district.  With  this  idea  in  mind  a  number  of  specimens  and  slides  from 
the  Penokee  district  have  been  examined  and  the  descriptions  given  by 
Vail  Hise  in  Monograph  XIX  carefully  read.  While  a  great  majority  of 
the  granule  structures  in  the  Gogebic  rocks  are  unquestionably  secondary 
concretions  resulting  from  the  alteration  of  iron  carbonate,  as  shown  by 
Van  Ilise,  a  considerable  number  of  granules  were  found  which  are  almost 
certainly  derived  from  greenalite  granules  (see  fig.  B  of  PI.  XVI),  for 
they  are  identical  in  every  way  with  the  derivatives  of  the  greenalite  gran- 
ules in  the  Mesabi  district. 

Ill  tills  coiiiicctiuii  it  is  of  interest  also  to  note  that  H.  L.  Smyth  found 


«  Described  and  figured  by  C.  E.  Van  Hise  in  Mon.  U.  S.  Geol.  Survey  Vol.  XIX,  1892. 


THE  BIWABIK  FORMATION.  119 

round  and  oval  forms  iu  the  feiTuginous  cherts  of  the  Groveland  formation 
(Lower  Huronian)  of  the  Felch  Mountain  area  in  Micliigan,  which,  on 
comparing  with  the  greenaHte  granules  and  their  derivatives  from  the 
Mesabi  district,  he  concluded  had  the  same  origin." 

The  matrix  of  the  chert  may  be  a  sparse  interstitial  filling  between  the 
granules,  or  it  may  form  most  of  the  rock  mass  and  contain-  but  few  isolated 
granules.  The  matrix  is  similar  to  that  of  the  unaltered  greenalite  rocks 
in  that  it  is  mainly  chert,  but  it  differs  in  containing  far  more  actinolite, 
griinerite,  cummingtonite,  iron  oxide,  calcite,  and  siderite,  and  rarely  epidote- 
zoisite  in  spherulitic  form.  Sometimes  also  green  chloritic  substances  are 
abundant,  either  irregularly  distributed  throug-h  the  matrix  or  forming  a 
definite  rim  about  the  granule.  In  the  latter  case  the  chlorite  is  in  part  in 
the  fibrous  form  known  as  delessite  (specimen  45173)  and  much  resembles 
uralite.  The  recrystallization  of  the  Tock  has  in  some  cases  made  the  chert 
in  the  matrix  coarser  than  that  of  the  granules  and  in  some  cases  the 
reverse.  The  leaching  out  of  the  carbonates  and  greenalite  from  the  matrix 
has  occasionally  left  cavities  which  give  the  pitted  character  to  the 
weathered  surface  of  the  cherts. 

Accompanying  the  recrystallization  of  the  chert  has  been  its  frequent 
adoption  of  radial  or  sheaf-like  forms,  giving  black  crosses  under  crossed 
nicols.  These  sheaves,  as  well  as  the  sheaves  of  actinolite,  griinerite,  and 
cummingtonite,  and  rarely  epidote,  frequently  lie  with  their  butts  against 
the  outlines  of  the  granules  and  send  theii-  points  outward  until  they 
interlock  with  similar  projections  from  adjacent  granules.  Commonly, 
also,  one  or  more  of  the  constituents  of  the  matrix  may  be  observed  to  lie 
partly  in  the  matrix  and  partly  in  the  granule,  thus  helping  to  obliterate  the 
granule.  Indeed,  undei-  crossed  nicols  the  g-ranules  may  not  be  observed, 
while  in  ordinary  light  their  position  may  be  indicated  by  the  distribution 
of  the  fine  pigment.     (See  figs.  C  and  I)  of  PI.  XV.) 

All  of  the  constituents  in  the  matrix  are  secondary  except,  perhaps,  a 
part  of  the  chert,  and  even  this  has  been  thoroughly  recrystallized.  The 
amphiboles  and  iron  oxide  may  be  observed  to  have  developed  by  the 
alteration  of  the  granules  and  some  of  the  lighter  amphiboles  by  the 
alteration  of  carbonate  and  chert  in  the  matrix.  The  carbonate  is  largely, 
though  not  entirely,  replacement  from  without,  for  it  may  be  observed 

«  Mon.  U.  S.  Geol.  Survey  Vol.  XXXVI,  1899,  pp.  421-122. 


120  THE  MESABI  IRON-BEARING  DISTRICT. 

replacing  nearly  all  the  other  constituents  of  the  rock  and  to  occur  in 
minute  veins  crossing  the  rock. 

The  segregation  of  the  iron  oxide  in  the  granules  and  matrix  due  to 
the  alteration  of  the  greenalite,  when  on  a  large  scale,  has  yielded  the  iron- 
ore  deposits,  and  this  feature  of  the  alteration  will  be  more  fully  discussed 
in  connection  with  the  origin  of  the  ores. 

The  above  description  covers  the  general  characteristic  features  of  the 
greater  part  of  the  cherts  of  the  iron  formation.  There  remain  certain  minor 
and  peculiar  phases  of  the  cherts  which  merit  further  brief  description. 

Chai'acteristic  of  basal  horizons  of  the  iron  formation  is  an  exceedingly 
dense  siliceous  and  ferruginous  rock,  with  gnarled  and  contorted  interband- 
ings  of  red,  white,  brown,  and  black  layers.  Some  of  the  quartz  is  vein 
quartz  which  fills  partings  essentially  parallel  to  the  layers.  (See  fig.  A  of 
PI.  XII.)  The  rock  in  general  has  the  hard  vitreous  aspect  of  jasper  of 
the  old  iron  ranges  of  Lake  Superior,  but  the  bands  are  not  nearly  so  clear 
cut  and  even  as  in  typical  jasper.  Under  the  microscope  the  rock  is  seen  to 
be  an  exceedingly  fine-grained  chert  containing  the  usual  granules,  which 
are  here  of  approximately  the  same  composition  as  the  groundmass,  but 
marked  off  by  iron  oxide  in  evenly  distributed  particles  or  in  concentric 
rings.  The  characteristic  feature  of  the  rock  is  the  matrix,  which  contains 
particles  of  iron  oxide  and  of  greenish  and  yellow  chloritic  and  ferrous  sili- 
cate substances,  and  occasional  axiolites  of  chalcedonic  quartz,  in  parallel 
gnarled  and  contorted  lines  resembling  the  flowage  lines  in  the  matrix  of  a 
vitreous  lava.     (See  fig.  B  of  PI.  XIII.) 

The  rock  in  places  grades  into  a  breccia,  and  is  with  exceeding  difficulty 
distinguished  from  certain  phases  of  true  conglomerates  containing  jaspery 
fragments  at  the  base  of  the  iron  formation.  Indeed,  the  similarity  is  so 
great  as  to  suggest  that  possibly  some  of  the  rocks  which  hove  been 
described  as  conglomerates  are  really  breccias.  However,  most  of  the  con- 
glomerates have  true  waterworn  quartzitic  matrix,  which  may  be  easily  dis- 
tinguislied  from  the  cherty  matrix  of  the  brecciated  rocks. 

The  presence  of  the  breccia  at  the  basal  horizon  of  the  chert  leads  one 
to  suspect  that  the  gnarled  and  contorted  nature  of  the  ferruginous  chert  at 
this  horizon  is  due  to  differential  movement  between  tlie  ferruginous  chert 
and  the  quartzite  during  the  folding  of  the  Upper  Huronian  series.  (See 
pp    178-180.)     Spurr   has  described   the   brecciation  of  the   ferruginous 


PLATE  X. 


121^ 


PLATE    X. 

FEKEUGINOUS   CHERT    OF   IRON-BEAEING    FORMATION. 

Fig.  a.- — Gray  ferruginous  chert.  Specimen  45027.  From  Chicago  mine  in  sec.  4,  T.  58  N., 
R.  16  W.  Natural  size.  This  is  one  of  the  characteristic  aspects  of  the  ferruginous  cherts  of  the 
iron  formation.  Under  the  microscope  iron  oxide  and  chert  can  be  seen  still  marking  the  shapes  of 
the  greenalite  granules.     Described  pp.  116-120. 

Fig.  B. — Ferruginous  chert.  Specimen  45588.  From  the  Mahoning  mine.  Natural  size.  The 
rock  shows  interbanding  of  chert  with  iron  oxide.     Described  pp.  116--120. 

122 


U.  S.  GEOLOGICAL   SURVEY 


MONOGRAPH   XLIII    PL.     X 


FERRUGINOUS  CHERT  OF  IRON-BEARING  FORMATION. 


PLATE    XI. 


123 


PLATE     XI. 

FERRUGINOUS   CHERT   OF   IRON-BEARING    FORMATION. 

Fig.  a. — Ferruginous  chert.  Specimen  45035.  From  Mountain  Iron  mine.  Natural  size.  This 
rock  is  a  dense  yellow  chert  'which  is  frequently  associated  with  iron-ore  deposits.  Under  the 
microscope  the  greenalite  granules  are  seen  to  be  represented  by  slightly  polarizing  fine-grained  chert, 
which  lies  in  a  matrix  of  limonite.     Described  pp.  137-138. 

Fig.  £. — FeiTUginous  chert.  Specimen  45309.  From  Diamond  mine  in  sec.  15,  T.  56  N.,  R.  24  W. 
Natural  size.  The  rock  shows  the  irregular  mottling  of  the  iron  oxide.  The  remains  of  the  greenalite 
granules  can  be  seen  under  the  microscope.     Described  pp.  116-120. 

Fig.  C. — Ferruginous  chert.  Specimen  45603.  From  Clark  mine.  Natural  size.  The  rock 
shows  interbanding  of  chert  with  iron  oxide.     Described  pp.  116-120. 

124 


MONOGRAPH    XLIII    PL.    XI 


-t; 


fc?- ■'?■**  v'^''^ '  *- -"  -  ■'^■s- .T  *  .,  '•- 


<K':;.-.  ■^■>, ;,,  :.'■'-  y 


(^■) 


'  L  ; 


FERRljniNJniJ?;   CHFRT   of   IRONRFARIMn    FORMATiriM. 


PLATE    XII. 


12.5 


PLATE    XII. 

FERRUGINOUS   CHERT,   "jASPERT"  PHASE,  AND  FERRUGrNOUS   CHERT  IN  CONTACT  WITH 
QUARTZITE    OF   IRON-BEARING    FORMATION. 

Fig.  a. — Ferruginous  chert  with  gnarled  and  contorted  banding.  Specimen  45420.  From  drift 
fragments  just  east  of  Mesaba  station  in  sec.  21,  T.  59  N.,  R.  14  W.  Natural  size.  This  phase  of 
ferruginous  chert  is  characteristic  of  the  basal  horizon  of  the  Biwabik  formation.  The  red  bands 
are  iron-stained  chert;  the  lighter  ones  are  chert  and  vein  quartz.  Under  the  microscope  the  shapes 
of  the  granules  can  be  seen  to  have  been  retained  by  chert  and  iron  oxide.     Described  p.  120. 

Fig.  B. — Ferruginous  chert  in  contact  with  quartzite  of  iron  formation.  Specimen  40952.  From 
the  Cincinnati  mine.  Natural  size.  The  chert  is  the  gnarled  and  contorted  phase  characteristic  of 
basic  horizons.  The  sharpness  of  its  contact  with  the  ferruginous  quartzite  is  to  be  noted.  In  some 
places  ferruginous  quartzite  and  chert  of  this  kind  are  minutely  interbanded  at  this  horizon. 
Described,  pp.  120  and  156. 

126 


U.  S.  GEOLOGICAL   SURVEY 


MONOGRAPH   XLIII    PL.   XII 


FERRUGINOUS  CHERT,  "JASPERY"  PHASE,  AND  FERRUGINOUS  CHERT  IN  CONTACT  WITH  QUARTZITE 

OF  IRON-BEARING  FORMATION, 


PLATE  XIII. 


127 


PLATE   XIII. 

PHOTOMICROGRAPHS  OF  FRESH  AND  ALTERED  GREEXALITE  GRANULES  AXD  FERRUGINOUS 

CHERT   CONCRETION. 

Fig.  a. — Greenalite  rock  with  bands  of  carbonate.  Specimen  45178,  slide  15652.  From  100  paces 
north,  500  paces  west  of  southeast  corner  of  sec.  22,  T.  59  X.,  R.  15  W.  Without  analj'zer,  x  50. 
Greenalite  granules  slightly  altered  to  griinerite,  iron  oxide,  and  chert,  stand  in  a  matrix  of  chert. 
On  the  left  side  of  the  figure  is  a  band  of  carbonate,  probably  largely  calcium  carbonate,  but  perhaps 
in  part  iron  carbonate.  Attention  is  called  to  the  irregular  nature  of  the  greenalite  granules  near  the 
contact  with  the  carbonate  band.  The  irregular  dark  fragments  in  the  carbonate  band  are  also 
greenalite  and  their  alteration  product  griinerite.     Described  pp.  101-115. 

Fig.  B. — Chert  with  granules  and  banding.  Specimen  45419,  slide  15706.  From  hill  just  east  of 
town  of  ilesaba.  Without  analyzer,  x  50.  This  is  a  phase  of  chert  which  is  typical  of  basal  horizons 
of  the  iron  formation.  The  rock  consists  essentially  of  chert.  Iron  oxide  occurs  outlining  the  altered 
granules,  and  also  occurs  in  contorted  lines  and  bands  representing  flow  lines  in  a  lava.  Described 
p.  120. 

Fig.  C. — Greenalite  granules.  Specimen  45765,  slide  16395.  From  Cincinnati  mine.  With- 
out analyzer,  x  40.  The  granules  are  for  the  most  part  unaltered,  and  are  dark  green,  light  green, 
or  yellow.  Some  of  them  show  alterations  to  iron  oxide  and  to  dark-green  chloritic  material.  Where 
altered  they  become  dark  brown,  black,  or  dark  green.  The  matrix  is  entirely  chert.  Evidence 
of  crusliing  is  to  Ije  observed  in  minute  cracks  ramifying  through  the  sUde.  Note  the  remarkable 
similarity  in  shapes  of  these  granules  to  those  of  the  green  granules  in  Clinton  ores,  illustrated  PI.  XXI. 

Fig.  D. — Concretionary  chert.  Specimen  40767,  slide  15415.  From  the  NE.  J  of  the  SE.  \  of  sec. 
35,  T.  59  X.,  R.  17  W.  With  analyzer,  x  100.  This  is  one  of  the  rare  normal  concretions  in  the 
iron  formation  of  the  Mesabi  district.  The  interior  is  a  single  grain  of  quartz.  This  is  surrounded 
by  concentric  layers  of  quartz  and  iron  oxide,  the  latter  somewhat  hydrated.  The  matrix  is  chert. 
This  structure  resembles  the  concretions  figured  by  Van  Hise  from  the  Gogebic  district  (see  PI.  XVI), 
and  is  believed  to  be  quite  different  from  the  greenalite  granules  figured  in  the  jireceiling  plates. 
Described  p.  118. 

128 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XLIII   PL.  XIII 


PHOTOMICROGRAPHS  OF   FRESH   AND   ALTERED  GREENALITE  GRANULES   AND   OF 
FERRUGINOUS   CHERT  CONCRETION, 


THE   MERIDEN   GRAVURE   CO. 


PLATE    XIV. 


MON   XLIII — 03 9  129 


PLATE    XIV. 

PHOTOMICROGRAPHS    OF    FERRUGINOUS    CHERT    GRANULES,    SHOWING   MOTTLING. 

Fig.  a. — Ferruginous  chert  with  mottled  granule.  Specimen  4.5628,  slide  15978.  From  near  the 
center  of  the  SW.  J  of  sec.  10,  T.  58  N.,  E.  19  W.  Without  analyzer,  x  125.  The  granule  here  shown 
is  composed  of  chert  and  reddish  iron  oxide.  The  chert  occurs  in  small  polygonal  blocks  separated 
by  the  oxide.  Each  of  the  chert  individuals  contains  in  its  interior  a  more  or  less  noticeable  nucleus 
of  iron  oxide.  The  matrix  is  chert.  A  similar  structure  has  been  noted  in  the  iron  ores  of  the  Ver- 
milion district  and  in  the  Clinton  ores.  In  the  latter  cases  the  mottled  structure  is  clearly  due  to  the 
replacement  of  a  shell  with  regular  structure.     Described  p.  117. 

Fig.  B. — Another  granule  in  the  same  slide,  showing  a  different  aspect  of  the  same  feature. 
Described  p.  117. 

130 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XLIII   PL,   XIV 


PHOTOMICROGRAPHS  OF  FERRUGINOUS  CHERT  GRANULES  SHOWING  MOTTLING. 


THE   MERIDEN   GRAVURE   CO. 


PLATE  XV. 


13.1 


PLATE    XV. 

PHOTOMICROGRAPHS   OF    FERRUGINOUS    CHERT    SHOWING    LATER    STAGES   OF   THE 
ALTERATION    OF    GREENALITE    GRANULES. 

Fig.  ^1. — Ferruginous  chert  with  granules.  Specimen  45063,  slide  15563.  From  near  center  of 
sec.  22,  T.  60  N.,  R.  13  W.  Without  analyzer,  x  50.  The  granules  are  outlined  and  in  part  replaced 
by  iron  oxide.  The  matrix  is  chert.  The  complex  nature  of  one  of  the  granules  is  to  be  noted,. 
Apparently  one  complete  small  granule  is  entirely  inclosed  in  another  large  one.  Desci  -ibed 
pp.  116-120. 

Fig.  B. — Griineritic  ferruginous  chert.  Specimen  45603,  slide  15974.  From  Clark  mine.  With 
analyzer,  x  50.  The  rock  consists  of  chert  and  iron  oxide  and  griinerite.  The  iron  oxide  is  a 
yellowish-brown  hydrated  variety,  which  is  with  difficulty  distinguished  from  the  griinerite.  The 
granules  have  been  entirely  obliterated.     Described  pp.  116-120. 

Fig.  C. — Ferruginous  chert  with  granules.  Specimen  45183,  slide  15657.  From  400  paces  north, 
35  paces  west,  sec.  28,  T.  59  N.,  E.  15  W.  Without  analyzer,  x  50.  The  rock  consists  almost  entirely 
of  chert  with  a  small  amount  of  iron  oxide.  The  granules  are  marked  by  a  light  pigment  which  on 
a  hurried  examination  would  be  scarcely  noticed.     Described  pp.  116-120. 

Fig.  D. — The  same  under  crossed  nicols.  The  cherty  nature  of  the  rock  is  here  shown,  and  the 
granules  are  quite  obscured  by  the  double  refraction  of  the  chert.     Described  pp.  116-120. 

132 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH  XLIII   PL.  XV 


m^ 


k 


'i'  ,^i±i^ 


(D) 


PHOTOMICROGRAPHS   OF   FERRUGINOUS   CHERT  SHOWING   LATER   STAGES  OF  THE 
ALTERATION  OF  GREENALITE  GRANULES. 


THE   MERIDEN   GRAVURE   CO, 


PLATE   XVI 


133 


PLATE    XVI. 

PHOTOMICROGRAPHS   OF    FERRUGINOUS    CHERT   OF   PENOKEE-GOGEBIC    DISTRICT. 

Fig.  a. — Concretionary  chert.  Specimen  9048,  slide  2886.  From  Penokee-Gogebic  district. 
Without  analyzer.  These  are  the  normal  concretions  from  the  Penokee-Gogebic  district  supposed 
by  Van  Hise  to  be  secondary  developments  during  the  alteration  of  an  iron  carbonate.  (PI.  XXII, 
fig.  1,  Mon.  U.  S.  Geol.  Survey  Vol.  XIX.     Described  on  pp.  227  and  228  of  Mon.  XIX.) 

Fig.  B. — Ferruginous  chert.  Specimen  9625,  slide  .3150.  From  Penokee-Gogebic  district. 
Without  analyzer,  x  60.  The  granules  here  shown  are  identical  in  aspect  with  granules  in  the  IMesabi 
iron  formation  which  can  be  shown  to  have  developed  from  greenalite.  (Reproduced  from  PI.  XXVII, 
flg.  2,  Mon.  U.  S.  Geol.  Survey  Vol.  XIX.) 

134 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XLIII   PL.  XVI 


=;.< 


■>!  •!.  I-*:! 


(B) 


PHOTOMICROGRAPHS  OF  FERRUGINOUS  CHERT  OF  PENOKEE-GOGEBIC  DISTRICT. 


-HE   MEfilOEM   GRAVURE  CO. 


PLATE  XYII. 


135 


PLATE    XVII. 

PHOTOMICROGKAPHS    OF     FERRUGINOUS    AND    AMPHIBOLITIC     CHERT    OF    IRON-BEARING 
FORMATION    NEAR    CONTACT   WITH    DULUTH    GABBRO. 

Fig.  a. — Actinolitic,  griineritic,  and  magnetitic  chert.  Specimen  4.5141,  slide  15621.  From  south- 
east of  center  of  sec.  17,  T.  60  N.,  R.  12  W.  Without  analyzer,  x  50.  This  rock  is  close  to  the  contact 
with  the  Duluth  gabbro,  and  shows  the  typical  alterations  characteristic  of  the  contact.  The  chert 
is  in  much  larger  particles  than  in  the  western  portion  of  the  range  away  from  the  contact.  (Com- 
pare with  PI.  XV. )  The  particles  fit  in  somewhat  regular  polygonal  blocks.  The  iron  oxide  is  mag- 
netite instead  of  hydrated  hematite,  and  there  is  present  actinolite  and  griinerite.  The  amphiboles 
are  in  small  quantity  in  the  slide  shown,  but  the  short  actinolite  needles  may  be  seen  inclosed  in  the 
quartz.     Described  pp.  159-161. 

Fig.  B. — Actinolitic,  griineritic,  and  magnetitic  chert.  Specimen  45147,  slide  15626.  From  east 
of  the  north  quarter  post  of  sec.  3,  T.  60  N.,  R.  12  W.  With  analyzer,  x  50.  This  is  still  nearer  the 
gabbro  contact  than  the  rock  figured  in  A  above,  and  shows  correspondingly  coarser  chert.  The  radial 
fibers  are  griinerite,  and  perhaps  in  part  cummingtonite,  which  project  into  the  quartz.  The  differ- 
ence in  the  shade  of  the  right  and  left  portions  of  the  photograph  show  but  two  large  particles  of  chert 
to  be  represented.     Described  pp.  159-161. 

136 


U.  S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XLIII    PL.   XVII 


if  *-::i 


,       -rf-     ■'•TV  WT     A-*"^ 


m; 


ri>'; 


PHOTOMICROGRAPHS  OF  FERRUGINOUS  AND  AMPHIBOLITIC  CHERT  OF  IRON 
BEARING  FORMATION  NEAR  CONTACT  WITH   KEWEENAWAN  GABBRO. 


THE   MERIOEN   UKAVURE   CO. 


THE  BIWABIK  FORMATION.  137 

cherts  as  due  to  the  sti'aiii  incident  to  the  change  of  vohime  during  the 
alterations  which  the  rock  has  undei'gone.  Certain  of  the  brecciated 
phases  in  higher  horizons  of  the  formation  might  be  thus  explained,  but 
the  persistent  belt  at  the  base  is  evidence  of  more  concentrated  movement 
along  one  horizon  than  could  be  attributed  to  chemical  strains. 

Another  phase  closely  associated  with  the  above-described  jaspery 
phase  and  characteristic  of  the  lower  horizon  of  the  iron-bearing  formation 
is  a  dense  purplish-red  chert  in  which  the  characteristic  granules  are  very 
slightly  differentiated  by  a  somewhat  lighter  red  or  purple  color.  Under 
the  microscope  the  rock  appears  as  a  fine-grained  chert  with  granules 
marked  by  iron  oxide,  largely  hematite  and  magnetite,  sometimes  ai-rauged 
peripherally.  The  rock  differs  from  the  jaspery  rock  above  described  only 
in  lacking  the  gnarled  and  contoi'ted  minute  bandings  and  in  containing 
less  clear  quartz. 

A  rare  rock  associated  with  the  ferruginous  chert  is  composed  of  inter- 
bauded  chert,  iron,  and  greenish-yellow  material  which  is  partly  chlorite, 
in  the  form  of  delessite,  and  partly  serpentine.  This  may  be  observed  in 
the  Fayal  mine. 

The  ferruginous  chert  in  several  places  exhibits  ellipsoidal  nodules 
with  their  greater  diameters  in  the  plane  of  bedding.  They  may  be  well 
observed  in  the  railway  approach  of  the  Oliver  mine.  They  reach  a  diam- 
eter of  6  inches,  although  commonly  they  are  smaller  than  this.  The 
nodules  consist  of  ferruginous  chert  similar  to  that  in  the  layers  adjacent, 
differing  only  in  being  more  massive  and  perhaps  somewhat  finer  grained. 
The  layers  apparently  do  not  continue  through  the  nodules.  Some  of  them 
abut  against  the  nodules  and  others  bend  slightly  in  passing  by.  The 
nodules  are  similar  to  those  in  the  overlying  Virginia  slate  and  to  those 
found  in  slates  and  cherts  in  general. 

Still  another  type  which  should  be  specially  mentioned  is  one  which 
frequently  occurs  in  the  neighborhood  of  iron-ore  deposits,  a  dense  yellow 
chert  owing  its  color  to  limonite.  (Fig.  A  of  PL  XL)  Under  the  micro- 
scope there  appear  granules  with  unusual  characters.  In  ordinary  light  they 
are  practically  colorless ;  under  cross  nicols  they  are  almost  isotropic,  but 
show  yellow  polarization  colors  in  minute  spots,  indicating  incipient 
crystallization  of  quartz  and  perhaps  other  minerals.  The  matrix,  at  first 
glance,  is  apparently  composed  entirely  of  yellow  limonite,  but    a   high 


138  THE  MESABI  IRON-BEARINCt  DISTRICT. 

power  reveals  in  addition  the  presence  of  abundant  gi'iinerite  or  actinolite 
in  typical  sheaf-like  and  radial  forms. 

The  ferruginous  cherts  rarely  contain  a  considerable  amount  of  iron 
pyrites.  The  "gold  mine,"  600  steps  west  of  the  southeast  corner  of  sec.  29, 
T.  60  N.,  R.  13  W.,  is  a  good  example.  The  ferruginous  chert  is  here  a 
dark-gray  and  black,  fine-grained,  siliceous  rock,  in  which  the  shapes  of  the 
greenalite  granules  can  be  easily  distinguished,  althougli  the  granules  have 
been  comjjletely  altered  to  chert.  The  iron  pp'ites  occurs  in  large  crvstals, 
replacing  all  the  other  constituents  of  the  rock,  and  also  as  a  filling  in  the 
interstices  between  the  gi'anules,  and  marking  the  outlines  or  even  partially 
replacing  the  granules.  The  iron  pyrites  in  this  case  has  crystallized 
during  or  subsequent  to  the  alteration  of  the  granules. 

An  occasional  rock  may  be  seen  to  consist  of  a  dense  felted  mass  of 
dark-green  and  brown  amphibole,  which  is  probably  griinerite  or  cumming- 
tonite,  interbanded  with  carbonate  of  iron  or  calcium,  or  containing 
carbonate  in  rhombs,  which  frequently  show  a  beautiful  zonal  alteration  in 
their  interiors. 

Chemically  the  cherts  show  wide  variation.  Comprising,  as  above 
described,  rocks  consisting  almost  entirely  of  silica,  rocks  consisting-  very 
largely  of  iron  oxide,  and  rocks  of  intermediate  kinds,  with  greath"  varying 
quantities  of  associated  minerals,  the  variety  of  results  in  the  analyses  listed 
below  is  to  be  expected.  The  analyses  in  the  following  table  are  of  cherts 
lacking  any  large  amount  of  amphibole.  The  analyses  of  the  chei'ts  rich 
in  amphibole  are  given  in  a  separate  table.  A  part  of  the  analyses  are 
incomplete.  Dotted  lines  indicate  that  the  substances  have  not  been  looked 
for,  not  their  absence. 


THE  BIWABIK  FORMATION. 

Analyses  of  ferruginous  chei'ts. 


139 


1. 

2. 

3. 

4. 

5. 

6. 

7. 

s. 

9. 

10. 

SiO^ 

63.  92 
None. 
31.13 
24.22 
3.13 
.49 
None. 

JTrace. 

.48 

1.12 

Trace. 

None. 

.05- 

.021 
None. 

.10 

32.56 
None. 
66.02 
46.45 

.30 
None. 

.18 
J  None. 

.32 
.90 
.16 

None. 
.12 
.052 

None. 
.14  . 

77.60 

80.93 

AI2O3 

Fe,0, 

Fe 

14.78 

9.69 

25.25 

3  43 

FeO 

MgO 

CaO 

.......J 

1 

NajO 

}   ■- 

1 

K2O... 

i 

HjO- 

H2O+ 

TiOj 

CO., 

PA 

P 

None. 

.006 

.021 

.020 

.023 

.026 

.062 

009 

SO3 

MnO 

Loss  on  ignition... 

11. 

12. 

13. 

!*• 

15. 

16. 

17. 

18. 

19. 

20. 

SiO 

45.49 

95.73 

63.68 

85.97 

.67 

11.40 

57.00 

1.43 

27.05 

61.57 
16.83 

5.S7 

86  35 

AI2O3 

78 

Fe,0, 



7  41 

Fe 

33.61 

53.53 

1.72 

19.53 

51.41 

FeO 

.90 
.02 
.01 
.01 
.01 

.30 

11.08 
2.02 
.40 
.397 
.113 

6.41 

3.44 

.01 

.12 

1.59 

4.70 

3  46 

MgO 

05 

CaO 

01 

Na^O 

12 

KjO     . 

None. 

01 

HjO—  . 

1 

01 

H2O+ 

/ 

TiOj  

CO2 

1.22 

P,0, 

.03 
.0129 

5.70 
2.49 

P 

.013 

.025 

Trace. 

.019 

SO3 

MnO 

Loss  on  ignition 

.91 

140  THE  MESABI  IRON-BEARING  DISTRICT. 

1.  Ferruginous  chert  below  horizon  of  ore  deposit  (specimen  40751);  from  sec.  28,  T.  58  N., 
R.  17  W.,  north  of  Virginia.     Analysis  by  Geo.  Steiger. 

2.  Ferruginous  chert  in  horizon  of  ore  deposits  (specimen  40744);  from  Oliver  mine.  Analysis 
by  Geo.  Steiger. 

3.  Ferruginous  chert  showing  red  granules  (specimen  45662) ;  from  pit  in  southwest  of  see.  3, 
T.  58  N.,  R.  17  W.     Analysis  by  E.  T.  Ailen. 

4.  Ferruginous  chert,  basal  phase  with  red  granules  (specimen  45688) ;  from  outcrop  on  Sparta  road 
just  east  of  Fayal  mine.     Analysis  by  R.  B.  Green. 

5.  Ferruginous  chert  (average  of  cores  in  several  holes  from  beneath  ore  deposit);  from  Donora 
mine.     Analysis  by  Lerch  Bros. 

6.  Ferruginous  chert  (specimen  45590);  from  beneath  ore  deposit  in  NE.  }  of  NE.  J  sec.  11, 
T.  57  N.,  R.  21  W.     Analysis  by  Lerch  Bros. 

7.  Ferruginous  chert  (specimen  45596);  from  just  above  slate  in  pump  shaft  of  Penobscot. 
Analysis  by  Lerch  Bros. 

8.  Ferruginous  chert  (specimen  45589);  from  north  wall  of  Mahoning  mine.  Analysis  by 
Lerch  Bros. 

9.  Ferruginous  chert  (specimen  45543);  from  northwest  of  Mesabi  Chief  mine.  Analysis  by 
Lerch  Bros. 

10.  Ferruginous  chert  (specimen  45672  B);  from  Donora  mine.     Analysis  by  Lerch  Bros. 

11.  Ferruginous  chert,  hard  bluish  gray,  seen  gi-ading  into  blue  and  black  ore  (specimen  45694); 
from  Biwabik  mine,  east  end.     Analysis  by  R.  B.  Green. 

12.  Ferruginous  chert  within  ore  deposit;  from  Adams  mine. 

13.  Ferruginous  chert,  green,  banded,  siliceous  (specimen  45653);  from  south  of  Virginia  along 
Duluth,  Missabe,  and  Northern  tracks.     Analj'sis  by  E.  T.  Allen. 

14.  Ferruginous  chert,  hard  yellow,  seen  gradipg  into  limonite  (specimen  45692);  from  Biwabik 
mine,  east  side.     Analysis  by  R.  B.  Green. 

15.  Ferruginous  chert,  yellow  (specimen  45603);  from  Clark  mine.     Analysis  by  H.  N.  Stokes. 

16.  Ferruginous  chert,  in  middle  horizon;  from  NE.  J  sec.  21,  T.  59  N.,  R.  14  W.  Analysis  by 
Lerch  Bros. 

17.  Ferruginous  chert  (specimen  65,  chem.  series  No.  237).  Analysis  by  C.  F.  Sidener,  for  J.  E. 
Spurr.     (See  Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  10,  p.  116.) 

18.  Ferruginous  chert  (specimen  107,  chem.  series  No.  243);  from  SW.  ^  of  SW.  }  sec.  2,  T.  58  N., 
R.  19  W.  Analj'sis  by  A.  J.  Hammond,  for  J.  E.  Spurr.  (See  Geol.  Nat.  Hist.  Survey  Minnesota, 
Bull.  No.  10,  p.  134.) 

~  19.  Ferruginous  chert,  top  of  Biwabik  formation  (specimen  101,  ch'em.  series  No.  239) :  from  SE.  J- 
of  NW.  i  sec.  18,  T.  58  N.,  R.  18  W.  Analysis  by  Alonzo  D.  Meeds,  for  J.  E.  Spurr.  (See  Geol.  Nat. 
Hist.  Survey  Minnesota,  Bull.  No.  10,  p.  148. ) 

20.  Ferruginous  chert,  Biwabik  formation  (specimen  27  A,  variety  1);  from  sec.  33,  T.  58  N.,  R. 
17  W.  Analysis  by  C.  F.  Sidener,  for  J.  E.  Spurr.  (See  Geol.  Nat.  Hist.  Survey  Minnesota,  Bull. 
No.  10,  p.  54. ) 

It  is  apparent  froiu  the  above  that  the  ferruginous  cherts  grade  from 
almost  pure  cherts  to  iron  ore.  The  percentage  of  phosphorus  is  uniformly 
lower  than  that  of  the  ores  (see  pp.  214-217).  The  ferric  iron  is  greatly  in 
excess  of  the  ferrous  iron;  calcium  and  magnesium  oxides  are  rare;  carbon 
dioxide  is  almost  entirelj^  absent.  Another  characteristic  feature  which  is 
not  emphasized  by  the  analyses  is  the  presence  of  organic  matter.  The  loss 
on  ignition  consists  partly  of  the  oxidation  of  organic  matter,  according  to 
chemists  who  have  handled  the  Jlesabi  ores  (see  p.  218).  The  content  of 
organic  matter  is  not  so  gi-eat  in  the  ferruginous  cherts  as  in  the  slates. 


THE  BIWABIK  FORMATION. 


141 


The   cherts  in    which  the  amphibole  constituent   is    abundant   show 
composition  different  from  tliat  of  the  cherts  above  analyzed. 

Analyses  of  amphihoUtic  clierts. 


SiOj  (total)  

SiO^  (soluble  in  HCl) 

AlA 

FeA 

FeO 

MgO 

CaO..^ 

NejO 

KjO... 

HjO- - 

H2O+ 

TiOj 

CO2 

PA 

SO, 


Specimen 
45028. 


Specimen 
45648. 


MnO 

BaO 

Carbon  in  organic  matter  . 


50.36 

18.42 

.64 

6.46 

I  32.  91 

3.94 

.23 

None. 

None. 

.27 

4.64 

None. 

None. 

None. 

None. 

None. 

None. 

None. 

.18 


83.82 

3.52 

.39 

4.46 

8.77 

None. 

.60 

None. 

None. 

.13 

1.37 

None. 

.72 

.02 


None. 


Specimen 
45649. 


23.94 

12.76 

6.00 

.5.81 

34.29 

5.05 

1.51 

.28 

None. 


.41 

18.03 

.11 


Specimen 
45689. 


44.10 

15.93 

1.05 

10.80 

28.73 

2.43 

.33 

None. 

None. 

.51 

2.47 

None. 

9.71 

.04 


None. 


a  Owing  to  presence  of  organic  matter  the  determination  of  ferrous  iron  is  probably  high. 

Specimen  45028;  from  old  Chicago  mine  in  NE.  \  of  SE.  \  of  sec.  4,  T.  58  N.,  E.  16  AV.  Analysis 
by  George  Steiger. 

Specimens  45648  and  45649;  from  pit  near  Duluth,  Missabe  and  Northern  track,  about  one-half 
mile  south  of  Virginia.     Analysis  \>y  George  Steiger. 

Specimen  45689;  from  Donora  mine,  near  the  east  side  of  sec.  28,  T.  59  N.,  E.  15  W.  Analysis  by 
George  Steiger. 

In  these  four  samples  the  amphibole  is  mainly  dark  brown  or  dark 
green,  with  inclined  extinction,  which  might,  from  its  microscopic  char- 
acter, be  common  hornblende,  cummingtonite,  or  gi-iinerite,  but  it  is  in 
part  also  colorless  or  nearly  colorless  actinolite.  The  lack  of  alumina 
shows  that  it  is  not  hornblende.  In  all  there  is  a  small  amount  of  iron 
oxide  to  be  seen  in  the  slide.  In  specimens  45649  and  45689  siderite  is 
abundant.  In  strong  hydrochloric  acid  a  considerable  amount  of  the  dark 
material  was  dissolved  out,  leaving   a  white  residue  in  specimens   45028 


142  THE  MESABI  IRON-BEARING  DISTRICT. 

and  45648,  and  a  dark  residue  in  specimens  45649  and  45689.  It  seems 
probable  that  the  dark  amphiboles  and  the  iron  oxide  are  the  substances 
which  are  mostly  dissolved  by  the  hydrochloric  acid.  The  analyses  give 
results  which  accord  with  the  microscopic  observations.  The  ferrous  iron 
present,  except  that  combined  with  the  carbon  dioxide  to  form  siderite, 
may  be  supposed  to  be  mainly  combined  with  silica  to  form  griinerite,  the 
magnesium  and  calcium  oxides  to  be  combined  with  the  silica  to  form 
actinolite,  or  the  ferrous  iron  and  the  magnesium  and  calcium  oxides  to  be 
combined  with  silica  to  form  cummingtonite.  In  three  of  the  analyses 
magnesia  is  present  in  much  higher  percentage  than  calcium  oxide."  In  the 
analyses  of  the  greenalite  rock  on  page  108  and  of  the  amphibolitic  slates 
on  pages  144-145  the  same  fact  may  be  noted.  The  principal  amphiboles 
containing  such  a  ratio  of  magnesium  oxide  to  calcium  oxide  are  antho- 
phyllite  and  cummingtonite,  both  of  which  are  essentially  silicates  of 
magnesium  and  ferrous  iron.  The  amphibole  in  the  Mesabi  rocks  does  not 
have  the  optical  properties  of  anthophyllite,  but  has  properties  ranging 
from  those  characteristic  of  actinolite  to  those  characteristic  of  griinerite  or 
cummingtonite.  The  high  proportion  of  magnesium  oxide  to  calcium 
oxide  would  indicate,  therefore,  that  the  dark-colored  amphibole  is  at  least 
in  part  cummingtonite.  The  above  aualy.ses  show  a  lower  percentage  of 
comljined  water  than  obtains  in  the  analyses  of  unaltered  greenalite  gran- 
ules discussed  on  preceding  pages,  and  it  is  apparent  that  the  development 
of  the  amphiboles  has  involved  dehydration  of  the  original  greenalite. 

The  direct  development  of  dark  amphibole  from  the  greenalite  may  be 
well  observed  in  specimen  45689,  where  the  change  from,  the  greenalite 
apparently  has  been  merely  a,  matter  of  the  fine  recrystallization  of  the 
original  substance.  The  change  is  scarcely  noticeable  in  ordinary  light,  and 
under  crossed  nicols  is  shown  only  by  the  low  double  refraction.  In  the 
slide  no  actinolite  is  to  be  observed.  The  matrix  is  chert.  The  carbon 
dioxide  is  supposed  to  be  combined  with  the  lime,  magnesia,  and  a  part  of  the 
ferrous  iron;  the  carbonate  can  be  observed  in  the  slide.  The  remainder 
of  the  ferrous  iron,  together  with  the  small  amount  of   magnesium  oxide 

"In  this  connection  it  in  of  intereat  to  note  that  an  analysis  of  a  •j;runorite-niaij;netite-sfliist  from 
the  Marquette  district,  given  by  Van  Hise,  shows  a  similar  high  proportion  of  magnesium  to  calcium. 
(Mon.  V.  S.  Geol.  Survey  Vol.  XXVIII,  p.  .338,  Analysis  No.  3.)  Also  an  analysis  of  griiuerite  by 
Lane  and  Sharpless  shows  a  high  percentage  of  magnesium  and  a  lack  of  calcium.  (Am.  Jour. 
8ci.,  3d  series,  Vol.  XLII,  1891,  p.  506.)  The  analyses  suggest  that  the  amphibole  may  be  more  closely 
allied  with  I'liinniingtonite  tlum  trriinerite. 


THE  BIWABIK  FORMATION.  143 

present,  may  be  supposed  to  be  combined  with  the  soluble  silica  shown 
in  the  analysis  to  form  the  dark  amphibole.  The  ferric  iron  and  water  are 
combined  to  form  limonite,  which  may  be  observed  in  the  slide.  This 
would  leave  17.60  per  cent  of  FeO,  and  15.93  per  cent  of  soluble  Si02.  On 
the  basis  of  100  the  proportions  are  FeO  52.5  per  cent,  SiOs  47.5  percent, 
which  shows  the  green  substance  to  be  closely  allied  to  griinerite.  These 
figures  show  a  proportion  of  ferrous  iron  and  silica  similar  to  that  shown  in 
the  analyses  of  the  unaltered  green  granules.  They  differ  in  showing  less 
combined  water.  This  similarity  is  in  accord  with  the  microscopic  obser- 
vation that  the  amphibole  has  developed  by  simple  recrystallization  of 
the  original  green  granules. 

SILICEOUS,    FEKKUGINOUS,    AND   AJIPHIBOLITIC    SLATES. 

Under  this  head  are  grouped  a  variety  of  slaty  rocks  which  are 
interstratified  with  the  other  phases  of  the  iron  formation.  They  include 
dense,  black,  dark-gray,  green,  or  reddish  rocks  with  a  tendency  toward 
conchoidal  fracture,  and  the  slaty  parting  poorly  developed  if  at  all ;  rocks 
showing  banding  of  dark-green,  black,  gray,  red,  or  brown  layers  parallel  to 
the  bedding,  and  a  well-developed  cleavage  parallel  to  the  same  structure ; 
gradational  varieties  between  these  two,  between  them  and  the  feiTUginous 
cherts,  and  between  them  and  the  iron  ores  (PI.  XVIII,  figs.  B  and  C). 
Any  of  them  may  be  hard  or  soft,  carbonaceous  or  noncarbonaceous,  fine 
grained  or  medium  grained. 

Under  the  microscope  the  slates  are  seen  to  contain  principally  cherty 
quartz,  iron  oxide,  either  hematite  or  magnetite,  usually  in  octahedra,  or 
some  hj^drated  oxide,  monoclinic  amphibole  which  may  be  griinerite, 
cummingtonite,  or  actinolite,  and  possibly  even  common  hornblende,  a  small 
amount  of  carbonate  of  calcium  or  iron,  a  little  zoisite,  and  possibly,  also, 
a  little  chlorite.  From  the  optical  properties,  and  from  the  analysis  of  the 
rock,  it  is  thought  that  the  amphibole  is  mainly  griinerite  and  cummingtonite. 
There  is  much  variation  in  the  relative  proportion  of  the  principal 
constituents.  Some  of  the  slates  consist  almost  entirely  of  fine  cherty 
quartz  with  subordinate  quantities  of  dark  amphibole  in  radial  aggregates 
or  in  irregular  masses  and  of  the  iron  oxides.  Others  are  composed 
mainly  of  iron  oxide,  showing  but  small  quantities  of  the  quartz  and  dark 
amphibole.     Others  are  composed  of  a  tangled  mass  of  yellowish,  brownish. 


144 


THE  MESABl  IRON-BEARING  DISTRICT. 


and  greenish  amphibole  fibers  containing  minute  particles  of  iron  oxide, 
silica,  and  other  subordinate  constituents.  The  griinerite  is  far  more 
abundant  than  the  actinolite.  The  banding  frequently  shown  in  a  specimen 
is  due  to  the  segregation  of  the  above-named  elements  into  layers.  •  While 
it  may  be  convenient  in  description  to  refer  to  this  or  that  slaty  rock  as  a 
ferruginous  slate,  a  siliceous  slate,  an  amphibolitic  slate,  or  an  actinolite 
slate,  depending  upon  the  relative  abundance  of  the  constituents,  usually 
all  three  constituents  are  present  in  one  rock,  and  the  rocks  are  really 
amphibolitic,  siliceous,  and  ferruginous  slates.  Perhaps  the  most 
characteristic  feature  of  the  slates  as  a  gi'oup  is  the  abundance  of  the  dark 
amphibole. 

Con-esponding  to  the  mineralogic  vai-iation  in  the  slates  there  is 
considerable  chemical  variation,  as  shown  by  the  following  partial  analyses 
of  most  of  the  phases  of  the  slaty  rocks.  Dotted  lines  indicate  that  the 
substances  have  not  been  looked  for,  not  their  absence. 

Aiialyses  o^ siliceous,  amphibolitic,  and  ferruginous  slates  within  the  Bnoahik 

fannation. 


1. 

2. 

3. 

4. 

5. 

(i.                  7. 

SiO., 

37.11 
2.41 

44.22 
6.39 

48. 16 

53.86 
9.14 

35. 12         37. 01 

AI2O3 

1.  72    1      2. 19 

Fe,0, 

17.  51 

Fe 

22.20 

24.53 

29.90 

15.90 

42.18         41.00 

FeO 

26.13 
3.70 

MgO 

CaO 

Na^O                                                    '        -  09 

K.,0 

.62 

.95 
2.57 

.22 
6.16 

.09 

H,0- 

H.,0-1- 

TiO 

COj           

12.60 
.05 

.14 
.04 

P,0, 

p 

.004 

.098 

.036 

.  022 

S03 

None. 
1.21 

MnO  . 

.11 

.17 

Mn 

c 

TO 

Vol 

1.30 

1.15 

THE  BIWABIK  FORMATION. 


145 


Analyses  of  siliceous^  amp?dholitic^  and  fey'ruginous  slates  within  the  JBiwabik 

formation — Continued. 


8. 

9. 

10. 

11.          ']          VI. 

13. 

SiO^                                     

12.93 

1 

23.80 

AI2O3                             

! 

7.95 

Fe,0,   

5.97 

Fe 

54.81 

3.88 

3.40 

FeO  .'. 

32.21 

MeO 

5.89 

CaO 

4.67 

NajO 

I 

.29 

K2O 

.18 

H.0--, 

4.28 

TiO 

CO, 

11.84 

P„0, -  -  - 

.012 

.14 
.06 

p :. 

.009 

.010 

None. 

SO3 

MnO 

Mn 

1 

Trace. 

C 

11.34 

Vol 

Loss  on  ignition  . . 

3.  35 

1 

1.  Specimen  45461,  from  Moss  mine.     Analysis  b}'  Geo.  Steiger. 

2.  Specimen  45591,  from  Penobscot  mine,  278  feet  beIoT\-  taconite.     Analysis  by  H.  N.  Stokes. 

3.  Specimen  45670,  from  sec.  7,  T.  58  N.,  R.  18  W.     Analysis  by  R.  B.  Green. 

4.  Specimen  45672  A,  from  Douora  mine.     Analysis  b}'  Lerch  Bros. 

5.  Specimen  45600,  from  near  southeast  corner  of  NE.  \  of  SW.  \  sec.  21,  T.  58  N.,E.  20  W. 
Analysis  by  H.  N.  Stokes. 

6.  Specimen  45645,  from  cut  south  end  of  ilountain  Iron  mine.     Analysis  by  A.  T.  Gordon. 

7.  From  cut  south  end  of  Mountain  Iron  mine.     Analysis  by  A.  T.  Gordon. 

8.  Specimen  45677,  from  Adams  mine.     Analysis  )5y  R.  B.  Green. 

9.  From  north  of  Little  Mesabi  exploration.     Analysis  b)-  Lerch  Bros. 
10.  Specimen  45672,  from  Donora  mine.     Analysis  by  Lerch  Bros. 

.    11.  Specimen  45625,  from  northeast  of  Buhl,     Analysis  by  H.  N.  Stokes. 

12.  Specimen  45678,  from  south  of  Spruce  mine.     Analysis  by  E.  T.  Allen. 

13.  Specimen  112  ( Chem.  Series  No.  240)  NE.  \  of  SE.  J  of  sec.  17,  T.  58,  R.  19  W.    Analysis  by  A.  D. 
Meeds,  for  J.  E.  Spurr.     (See  Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  10,  p.  10) . 

The  essential  features  above  shown  are  the  variation  in  metallic  iron, 
the  considerable  content  of  ferrous  iron  as  compared  with  the  ferric  iron, 
low  alumina  as  compared  with  true  slate,  but  hig-h  as  compared  with  the 
other  rocks  of  the  iron  formation,  and  the   greatly  varying-  quantities  of 

MON  XLIII — OS 10 


146  THE  MESABI  IR0N-BEARI>;G  DISTRICT. 

carbon  dioxide  and  carbon,  both  of  them,  however,  fairly  abundant.  The 
lai'ge  proportion  of  mag-nesium  oxide  and  calcium  oxide  accords  well 
with  the  microscopic  determination  of  some  of  the  dark  amphibole  as 
cummingtonite. 

In  texture  and  in  mineralogic  and  chemical  composition  the  slaty 
rocks  of  the  iron  formation  differ  from  true  black  roofing  slates,  such,  for 
instance,  as  those  from  Vermont.  The  cleavage  is  not  as  good  as  in 
roofing  slates,  and  what  there  is  of  it  is  parallel  to  the  bedding  and  largely 
conditioned  bv  the  bedding',  and  not  by  the  deformation  of  the  rocks. 
Griinerite  is  abundant,  while  in  typical  roofing  slates  micaceous  and 
chloi'itic  constituents  are  important.  Finally,  the  percentage  of  iron,  and 
in  some  case^,  the  percentage  of  silica,  is  higher,  and  the  percentage  of 
alumina  is  much  lower  than  in  typical  slates.  The  slates  in  the  iron  forma- 
tion also  differ  from  the  overlying  Virginia  slates  in  a  manner  described  in 
connection  with  the  latter  (see  p.  176). 

Some  of  the  typical  occurrences  of  slate  within  tlie  iron  formation 
from  which  specimens  have  been  collected  are  specified  below : 

The  pump  shaft  of  the  Penobscot  mine  in  sec.  1,  T.  57  N.,  R.  21  W"., 
passes  through  278  feet  of  ferruginous  chert  and  bottoms  in  slate  (speci- 
mens 45591  to  45593). 

Northeast  of  the  center  of  sec.  27,  T.  58  N.,  R.  20  W.,  E.  J.  Longear 
drilled  through  5(i  feet  of  iron-formation  material,  mainly  ferruginous  chert, 
to  slate  (specimen  45600). 

Test  pit  in  the  NE.  i  of  SE.  i  sec.  17,  T.  5S  N.,  R.  19  W.  (specimen 
45630). 

Drill  hole  hi  the  NE.  i  of  NW.  \  sec.  20,  T.  58  N.,  R.  19  W.  (specimen 
45541). 

Test  pit  in  NE.  4  of  SW.  i  sec.  10,  T.  58  N.,  R.  19  W.  (specimen  45625). 

Test  pit  just  southeast  of  northwest  corner  of  sec.  8,  T.  58  X.,  R.  18  W. 
(specimens  45639  and  45640). 

Test  pit  and  drill  hole  in  the  NW.  \  of  NW.  ^  sec.  7,  T.  58  N.,  R.  18 
W.  (specimen  45670). 

Test  pit  south  of  A^irginia,  west  of  the  Duluth,  Missabe  and  Northeni 
track  in  the  SE.  \  of  SW.  \  sec.  H,  T.  58  N.,  R.  17  W.,  near  contact  with 
the  overlying  Virginia  slate  (specimen  45652). 

Test  ])it  just  north  of  the  old  Norman  open  pit  in  the  SE.  \  of  NW.  \ 
sec.  9,  T.  58  N.,  R.  17  AV.  (specimens  40741  and  40742). 


THE  BIWABIK  FORMATION.  147 

Test  pits  south  of  the  Spruce  mine  in  NW.  \  of  NE.  {  sec.  6,  T.  57 
N.,  R.  17  W.  (specimen  45678). 

Drill  hole  in  Fayal  mine.  Slate  reached  under  200  feet  of  ferruginous 
chert  (specimen  45734). 

Test  pit  just  south  of  the  Elba  mine  near  the  north  line  of  the  NE.  ^ 
of  NE.  i  sec.  24,  T.  58  N.,  R.  17  W.  (specimen  40863). 

Test  pits  and  shafts  of  the  old  Chicago  mine  in  the  NE.  ^  of  SE.  ^ 
sec.  4,  T.  58  N.,  R.  16  W.,  near  contact  with  overlying  Virginia  slate. 

Test  pits  in  the  Cincinnati  mine  in  the  SW.  J  of  NE.  ^  sec.  2,  T.  58  N., 
R.  16  W.,  and  the  SE.  4  of  NW.  i  sec.  2,  T.  58  N.,  R.  16  W.,  near  contact 
with  overlying  slate  (specimen  45039). 

Test  pit  and  drill  hole  in  SE.  i  of  NE.  i  sec.  28,  T.  59  N.,  R.  15  W., 
the  little  Mesabi  exploration  (specimen  45672). 

Test  pits  in  the  SE.  i  of  SE.  i  sec.  22,  T.  59  N.,  R.  15  W.  (specimen 
45176). 

Test  pit  in  SW.  i  of  SW.  \  sec.  26,  T.  59  N.,  R.  15  W.  (specimen 
45737). 

Test  pit  in  SE.  i  of  NW.  i  sec.  28,  T.  59  N.,  R.  15  W.  (specimen 
45191  calcareous). 

Drill  hole  in  the  SE.  i  of  NE.  i  sec.  19,  T.  59  N.,  R.  14  W.  (speci- 
men 45003). 

Drill  hole  in  the  SW.  i  of  SW.  i  sec.  16,  T.  59  N.,  R.  14  W.  (speci- 
men 45006). 

Test  pit  in  the  NW.  i  of  SW.  ^  sec.  21,  T.  59  N.,  R.  14  W.  (specimens 
45009  and  45010). 

Test  pits  in  SW.  i  of  SW.  i  sec.  15,  and  NW.  i  of  NW.  J  sec.  22, 
T.  59  N.,  R.  14  W.  (specimens  45224  and  45228). 

Exposure  (?)  NE.  i  of  SE.  i  sec.  21,  T.  59  N.,  R.  14  W.  (specimen 
45699). 

Exposure  ("?)  east  of  the  west  quarter  post  of  sec.  22,  T.  59  N.,  R.  14 
W.  (specimen  45700.) 

From  near  the  east  side  of  range  14  eastward  through  ranges  13  and 
12  the  slates  interbedded  with  iron  formation  are  in  great  abundance,  and, 
because  of  the  good  exposure  of  the  rocks  in  this  area,  can  be  well  observed. 
One  may  mention  in  particular  the  belt  extending  north  and  east  of  Mall- 
man  camp  (see  map,  PI.  VI),  and  the  slate  along  the  road  in  the  NE.  \  of 
sec.  1,  T.  59  N.,  R.  14  W. 


148  THE  MESABl  lEON-BEARING  DISTRICT. 

It  has  been  maiutaiiied  by  certain  of  the  mining  engineers  on  the  Mesabi 
range  that  the  slates  found  within  the  area  of  the  iron  formation  represent, 
in  large  part,  patches  of  the  Virginia  slate,  which,  in  these  particular  places, 
have  been  left  as  islands  during  the  erosion  which  removed  most  of  the 
slate  from  the  area.  The  area  is  heavily  drift  covered,  and  the  slates  are, 
for  the  most  part,  found  in  isolated  explorations  and  show  no  relations  to 
surrounding  rocks.  It  is  possible  that  the  occurrence  of  some  small  part 
of  the  slate  may  be  explained  in  this  way,  but  it  is  believed  that  practically 
all  of  it  is  interstratified  with  the  iron  formation,  for  the  following  reasons: 

(1)  In  a  number  of  places  explorations  have  gone  through  the  iron 
formation  into  the  slate,  and  then  into  iron  formation  again,  xilso,  in  the 
east  end  of  the  district  actual  interbedding  can  be  observed  in  exposures. 

(2)  As  the  iron  formation  and  Virginia  slate  are  conformable  and  have 
the  same  dip,  patches  of  the  Virginia  slate  left  on  top  of  the  iron  formation, 
especially  where  well  to  the  north,  must  necessarily  have  a  considerably 
higher  elevation  than  the  surrounding  rock  surface.  So  far  as  can  be  ascer- 
tained under  the  thick  covering  of  drift,  the  slates  in  the  iron  formation  are 
not  elevated  above  the  siirrounding  rock  surface. 

(3)  Commonly,  wdiere  exploration  has  gone  far  enough,  the  slate  in 
the  iron  formation  is  found  to  come  to  the  rock  surface  in  a  narrow  band, 
with  strike  parallel  to  the  strike  of  the  iron  formation  layers,  and  not  in  an 
in-egular  patch  witli  outlines  determined  by  erosion. 

(4)  In  mineralogic  and  chemical  composition  the  slate  layers  within 
the  iron  formation  are  somewhat  different  from  the  Virg'inia  slate.  (See 
p.  176.) 

If,  is  not  improbable,  indeed  it  is  likely,  that  certain  of  the  interstrati- 
fied th  te  layers  in  the  iron  formation  have  been  mapped  as  Virginia  slate; 
that  is,  where  full  data  have  been  absent  the  Virginia  slate  boundary  has 
been  extended  to  the  north  to  cover  slate  shown  in  some  exploration  which 
may  really  !.  3  a  layer  of  slate  interstratified  in  the  iron  formation.  If  this 
is  true,  future  wo.]:  will  resvilt  in  carrying  the  Virginia  slate  boundary 
farther  south. 

As  ore  deposits  have  not  been  found  to  occur  under  the  solid 
Virginia  slate,  and  as  the  slate  layers  in  the  iron  formation  have  an  influence 
on  the  concentration  of  the  ore  deposits  hiter  discussed,  it  is  apparent  that  the 
discrimination  of  tlie  various  slates  is  a  niattcr  of  importance  to  mining  men. 


THE  BIWABIK  FORMATION. 


149 


PAINT   KOCK. 


Throughout  the  iron  formation,  and  particularly  adjacent  to  the  ore 
deposits,  are  thin  seams  of  paint  rock,  which  have  resulted  from  the  altera- 
tion of  the  slates  above  described.  The  paint  rocks  are  essentially  soft 
red  or  yellow  or  white  clay.  They  retain  the  original  bedding  of  the  rocks 
from  which  they  were  derived,  the  structure  being  marked  by  alternation 
of  bands  of  different  color.  In  situ  the  paint  rocks  are  moist  and  soft. 
When  taken  out  and  dried  they  become  harder,  but  retain  a  soft,  greasy  feel. 

The  alteration  of  the  paint  rocks  from  slates  is  proved  by  the  numer- 
ous intermediate  phases  to  be  observed.  (See  PL  XVIII.)  At  several  mines 
also  the  paint  rocks  associated  with  the  ore  deposits  are  in  the  same  horizon 
as  slates  immediately  adjacent.  At  the  Penobscot  mine  paint  rock  forms  a 
persistent  horizon  near  the  bottom  of  the  mine,  while  the  pump  shaft  sunk 
in  the  adjacent  rock  struck  slate  at  the  same  horizon.  At  the  Biwabik 
mine  there  is  a  capping  of  paint  rock  over  the  ore,  which,  according  to  the 
superintendent,  has  been  found  by  test  pitting  to  grade  southward  into  the 
true  black  slate  mapped  as  Virginia  slate.  South  of  the  exploration  of 
the  Medow  Mining  Company  in  the  NW.  ^  of  sec.  3,  T.  58  N.,  R.  15  W., 
a  considerable  quantity  of  paint  rock  is  encountered  along  the  north  mar- 
gin of  the  Virginia  slate.  Other  instances  might  be  cited,  but  the  proof 
is  so  conclusive  at  these  places  that  further  evidence  is  hardly  necessary. 
Chemically,  the  paint  rocks  have  the  characteristics  shown  in  the  following 
partial  analyses: 

ATwlyses  of  paint  rock. 


1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

SiOa   

9.54 
7.00 

77.30 
.45 

55.57 

3.28 

36.57 

32.16 
31.39 

20  94 

A1,0, 

6.58 

7.75 

7.62 

7.23 

19  01 

FsjOa 

FeO 

Fe 

25.60 

54.20 

47.49 

47.48 

56.72 

17.56 

30  88 

TiOj 

.61 

\ 

HjO  - 

9.55 

13  40 

H2O  + 

/ 

None. 
Trace. 

CO2 

PA - 

.20 

p 

.055 

150  THE  MESABI  IRON-BEARING  DISTRICT. 

1.  Paint  rock  (specimen  40661)  from  Maiioning  mine.     Analysis  b)' George  Steiger. 

2.  Paint  rock  (specimen  4.5.594)  from  Penobscot  mine,  beneath  ore.     Analysis  by  H.  N.  Stokes. 

3.  Paint  rock  from  Franklin  mine.     Analysis  by  E.  F.  Johnson. 

.  4.  Paint  rock  from  Wacoutah  property  near  Mountain  Iron.     Analysis  by  Lerch  Bros. 

5.  Paint  rock  from  Wacoutah  property  near  Mountain  Iron.     Analysis  by  Lerch  Bros. 

6.  Paint  rock  from  Wacoutah  property  near  Mountain  Iron.     Analysis  by  Lerch  Bros. 

7.  Light  portion  of  banded  red  and  white  paint  rock  (specimen  45646)  from  Mountain  Iron  mine. 
Analysis  by  A.  T.  Gordon. 

8.  Dark  portion  of  banded  red  and  white  paint  rock  (specimen  45646)  from  Mountain  Iron  mine. 
Analysis  by  A.  T.  Gordon. 

SIDERITIC    AND    CALCAREOUS    ROCKS. 

Associated  with  the  slaty  layers  iu  the  iron  formation,  and  particularly 
with  the  greenalite  rocks,  are  carbonates  of  iron  and  calcium  in  small 
quantity  Most  of  the  carbonate  reacts  readily  with  cold  dilute  hydro- 
chloric acid  and  is  certainly  limestone,  which,  from  the  aualvsis  of  rocks 
containing  it,  is  doubtless  magnesian.  Some  of  the  carbonate,  how- 
ever, is  certainly  siderite,  as  shown  by  the  analysis  on  page  141.  The 
carbonates  occur  in  minute  clear-cut  layers  interbedded  with  the  iron 
formation  (see  PI.  XIII,  fig.  A),  in  veins  cutting  across  the  bedding,  iu 
irregular  aggregates  in  iron  formation  layers,  and  in  well-defined  rhombo- 
hedral  crystals  in  the  same.  In  the  carbonate  bands  are  small  quantities 
of  iron  oxide,  ferrous  silicate,  and  chert,  and  in  the  bauds  of  these  minerals 
are  small  quantities  of  the  carbonate.  In  some  cases  the  carbonates  are 
coarsely  crystalline  and  fresh  and  clearl}-  have  resulted  from  the  replace- 
ment of  the  other  constitutents  in  the  rock,  particularly  the  ferrous  silicate, 
or  from  infiltration  along  cracks  and  crevices.  In  other  cases,  especiall}' 
where  in  distinct  layers  interbedded  with  unaltered  ferrous  silicate  phases 
of  the  formation,  the  carbonate  layers  seem  certainly  to  be  original.  At 
the  top  of  the  iron  formation  and  closely  associated  with  the  basal  horizons 
of  the  Virginia  slate  are  several  feet  of  clear  calcium  carbonate,  which  is 
described  in  connection  with  the  Virginia  slate. 

North  of  Birch  Lake  is  a  siliceous  and  sideritic  slate  exposed  in 
one  pit.  (See  fig.  7,  p.  184.)  The  carbonate  is  a  peculiar  grayish  banded 
slate,  which  near  the  surface  and  adjacent  to  cracks  is  weathered  to  a 
rusty-brown  color.  The  weathering  penetrates  several  inches  from  the 
surface.  Under  the  microscope  the  rock  is  seen  to  be  largely  made  up 
of  carbonate  in  bands  and  in  isolated  rhombs  associated  with  magnetite 
and  chert.  The  rusty  weathei'ing  of  the  carbonate  and  the  fact  that  it 
effervesces  but  slightly,  if  at  all,  with  cold  hydrochloric  acid  indicate  it  to 


PLATE  XVIII. 


151 


PLATE    XVIII. 

SLATE,    FERRUGINOUS    SLATE,  AND   PAINT   ROCK   IN    IRON    FORMATION    AND    CONTACT    OF 
IRON-BEARING   FORJIATION   AVITH   INTRUSIVE    GRANITE. 

Fig.  .1. — Contact  of  Biwabik  formation  and  Embarrass  granite.  Specimen  45138.  From  the 
NW.  }  sec.  17,  T.  60  N.,  K.  12  W.  Natural  size.  The  granite  is  in  intrusive  contact  with  the  iron 
formation.     Note  the  purple  quartz  phenocrysts  in  both.     Described  pp.  186-188. 

Fig.  B. — Banded  slate.  Specimen  45592.  From  Penobscot  mine,  298  feet  below  ferruginous  chert. 
Natural  size.     Described  pp.  14.3-148. 

Fig.  C. — Banded  ferruginous  slate.  Specimen  45594.  From  Penobscot  mine,  298  feet  below  ferru- 
ginous chert.     Natural  size.     Described  pp.  143-148. 

Fig.  D. — Paint  rock.  Specimen  45587.  From  north  wall  of  the  Mahoning  mine.  Natural  size. 
The  derivation  of  the  paint  rock  from  the  alteration  of  slate  is  evident.  ^lany  specimens  have  been 
collected  showing  complete  gradation  between  the  two.     Described  pp.  149-150. 

152 


U.  S,  GEOLOGICAL   SURVEY 


MONOGRAPH    XLIII    PL.    XVIII 


SLATE,  FERRUGINOUS  SLATE,  AND  PAINT  ROCK  OF  IRON-BEARING  FORMATION,  AND  CONTACT  OF 
IRON-BEARING   FORMATION  WITH   INTRUSIVE  GRANITE. 


THE  EIWABIK  FORMATION.  153 

be  au  iron  carbonate  or  siderite.  The  siliceous  phase  of  the  slate  is  clearly 
a  replacement  of  the  carbonate.  Indeed  in  hand  specimens  it  is  almost 
identical  in  appearance  It  is  a  minutely  banded  gray  and  black  and 
brown  rock  with  a  poor  parting  parallel  to  the  banding.  Under  the 
microscope  it  is  seen  to  have  a  medium-grained,  quartzose  background, 
in  which  are  octahedra  of  magnetite.  The  concentration  of  the  iron  oxide 
and  the  quartz  in  alternate  bands  gives  the  minute  banding  observed  in 
the  hand  specimen. 

An  analysis  of  the  rock  (specimen  45161)  by  George  Steiger  is  as 
follows: 

Analysis  of  siliceous  slate  of  iron  fmvnation  at  contact  of  gahhro. 

Per  cent. 

SiOj 78. 95 

AljOg None. 

FeA 13.89 

FeO ] .  23 

MgO 18 

CaO 81 

NajO None. 

KjO None. 

H,0- 73 

H^O-h 2.21 

TiOj None. 

COa 1.59 

P2O5 04 

SO3 None. 

MnO 11 

Total 99.  74 

The  rock  analyzed  is  from  the  same  mass  as  the  carbonate,  and  before 
analysis  was  supposed  itself  to  be  partly  carbonate. 

The  total  absence  of  any  traces  of  greenalite  granules  and  the  occur- 
rence of  the  carbonate  in  well-bedded  layers,  evidently  little  altered,  make 
it  probable  that  the  sideritic  slate  at  Birch  Lake  is  original.  It  is  the 
nearest  approach  in  nature  and  abundance  to  the  original  sideritic  slates 
characteristic  of  the  Penokee-Gogebic  range  to  be  anywhere  seen  on  the 
Mesabi  range.  Eastward,  in  the  neighborhood  of  Gunflint  Lake,  structures 
characteristic  of  the  alteration  of  siderite  are  to  be  observed  Its  occur- 
rence at  the  east  end  of  the  district,  together  with  certain  structures  in  the 
Gunflint  Lake  region  to  the  east  characteristic  of  altered  sideritic  rocks,  is 
suggestive  of  a  gradation  toward  the  east  to  phases  characteristic  of  the 
Penokee-Gogebic  iron  formation. 


154  THE  MESABI  IROX-BEARING  DISTRICT. 

CONGLOMERATES   AND   QUARTZITES. 

At  the  base  of  the  iron  formation  is  a  thin  laver  of  fairly  coarse 
fragmeutal  Jiiaterial  consisting  in  places  of  conglomerate  alone  and  in 
other  places  of  conglomerate  and  quartzite.  At  several  localities  the  fnll 
succession  from  the  iron  formation  above  through  the  quartzite  or 
conglomerate,  or  both,  into  the  Pokegama  quartzite  below  may  be  studied. 
In  the  SW.  1  of  SW.  i  of  sec.  3,  T.  58  N.,  R.  17  ^Y.  (see  fig,  3,  p.  89)  are 
a  considerable  number  of  test  pits  which  have  either  been  bottomed  in  the 
quartzite  belonging  to  the  iron  formation  or  have  gone  through  this  and 
have  been  bottomed  in  the  Pokegama  quartzite.  At  the  Fayal  and  Adams 
mines  green  quartzites  are  found  beneath  the  ferruginous  chert,  and  so 
closely  resemble  it  that  they  were  classed  as  "taconite"  by  the  drillers. 
Drill  holes  on  the  NE.  \  of  the  SE.  \  of  sec.  4,  T.  58  N.,  R.  16  W.,  put 
down  by  E.  A.  Sperry,  penetrated  the  iron-formation  conglomerate 
(45753),  Pokegama  quartzite,  and  Lower  Huronian  rocks,  in  the  order 
named.  One  of  the  northward  drifts  of  the  Cincinnati  mine  penetrates 
the  quartzite  at  the  base  of  the  iron  formation.  A  little  southwest  of  tlie 
center  of  sec.  18,  T.  59  N.,  R.  14  W.,  is  a  trench  in  which  may  be  observed 
the  actual  contact  of  the  iron  formation  and  the  Pokegama  quartzite. 
Here  only  the  conglomerate  appears  at  the  base  of  the  iron  formation. 
In  the  S.  i  of  SE.  \  of  sec.  29,  T.  60  N.,  R.  13  W.,  the  same  thing  may  be 
observed  in  natural  exposure.  Here  also  the  conglomerate  alone  is  present 
at  the  base  of  the  iron  formation. 

A  di-ill  hole  put  down  by  E.  J.  Longear  in  the  SW.  4  of  SE.  \  oi  sec. 
32,  T.  59  N.,  R.  17  W.  penetrated  iron  formation,  quartzite,  conglomerate, 
and  Pokegama  quartzite  in  the  order  given.     (Specimens  40851  to  40855.) 

Thin  films  of  conglomerate  lie  on  the  upper  surface  of  the  Pokegama 
quartzite  a  little  north  of  the  east  quarter  post  of  sec.  13,  T.  58  N., 
R.  17  W.;  just  north  of  the  Arcturus  mine  near  the  center  of  sec.  13, 
T.  56  N.,  R.  24  W.;  at  the  falls  of  the  Prairie  River  in  range  25;  and 
at  Pokegama  Falls  in  range  26. 

On  the  dumps  of  a  number  of  the  pits  adjacent  to  the  northern 
boundary  of  the  iron  formation  are  found  conglomerate  and  quartzite  with 
such  n'lati<ins  to  the  iron  formation  and  the  Pokegama  quartzite  as  to  show 
them  to  be  basal  to  tlic  iron  formation.     Some  of  these  pits  are  located  as 


THE  BIWABIK  FORMATION.  155 

follows:  The  NW.  i  of  SE.  \  sec.  13,  T.  58  N.,  R.  17  W.  (specimens  46020 
and  46021);  in  the  NE.  {  of  SW.  i  sec.  33,  T.  58  N.,  R.  17  W.  (specimen 
46026);  in  the  SE.  i  of  SE.  i  sec.  5,  T.  58  N.,  R.  16  W.  (specimen 
46034);  in  the  SW.  i  of  SW.  I  sec.  4,  T.  58  N.,  R.  16  W.  (specimens 
46031  and  46032). 

Finally  numerous  fragments  of  conglomerate,  similar  to  the  conglom- 
erates at  the  base  of  the  iron  formation,  were  found  in  the  drift  in  such  a 
position  as  to  show  the  probability  of  their  having  been  carried  to  their 
present  resting  place  from  the  contact  of  the  iron  formation  and  the 
Pokegama  quartzite. 

The  quartzite  is  best  exhibited  in  the  pits  in  the  SW.  ^  of  SW.  ^  sec. 
3,  T.  58  X.,  R.  17  W.  (fig.  3,  p.  89),  and  a  description  of  its  character  and 
occurrences  here  will  suffice  for  the  district.  Several  pits  were  cleaned 
out,  and  the  succession  carefully  studied.  In  going  down  the  first  rock 
struck  was  a  massive,  somewhat  vitreous,  fine-grained  quartzite,  with 
green,  brown,  red,  and  yellow  colors.  The  green  quai'tzite  is  evidently 
the  original  form,  and  the  red,  brown,  and  yellow  colors  are  due 
to  iron  staiifing  or  to  bleaching.  Microscopically,  the  green  rock  is 
observed  to  be  composed  of  rounded  and  subangular  grains  of  quartz  and 
a  subordinate  amount  of  greenish-yellow  granules  of  ferrous  silicate  like 
those  in  the  ferruginous  cherts  above  described.  The  matrix  is  composed 
of  a  dense  greenish  chloritic  or  ferrous  silicate  substance  stained  with  iron 
ore.  The  altei-ation  of  this  cement  to  iron  oxide  or  the  leaching  out  of  its 
ferrous  silicate  constituent  has  given  the  red,  brown,  and  yellow  quartzites. 

Beneath  10  or  12  feet  of  this  kind  of  quartzite  is  a  heavily  ferruginous 
quartzite  which  is  coarser,  contains  better  rounded  grains,  a  heavily 
ferruginous  matrix,  and  is  in  places  considerably  softer.  The  well-rounded 
quartz  grains  stand  out  like  eyes  in  the  abundant,  heavily  ferruginous 
matrix.  At  certain  of  the  pits  the  content  of  iron  is  so  great  that  the 
rock  looks  like  a  soft  ore,  and  indeed  analysis  shows  it  to  come  almost 
within  the  limit  of  merchantable  ore.  Under  the  microscope  the  quartz 
grains  are  seen  to  be  well  rounded,  usually  to  show  pressure  effects 
by  undulatory  extinction,  and  sometimes  apparently  to  be  themselves 
composite — that  is,  to  consist  of  several  grains  as  thougii  derived  from  a 
preexisting  quartzite.  In  addition  to  the  quartz,  there  are  fairly  numerous 
greenalite  granules  with  characteristic  greenish-yellow  colors  and  showing 


156 


THE  MESABI  IRON-BEARING  DISTRICT. 


considerable  alteration  to  hematite,  and  small  rounded  grains  composed 
of  cherty  quartz,  sometimes  with  radial  forms  perhaps  representing  the 
alteration  of  the  greenalite  granules.  The  matrix  is  yellowish  or  reddish 
brown  and  consists  mainly  of  hematite  with  a  slight  intermixture  of 
greenish-yellow  chloritic  or  ferrous  silicate  matei'ial.  The  heavil)^ 
ferruginous  quartzite  is  but  a  few  feet  thick.  Partial  analyses  of  both 
the  green  and  heavily  feiTUginous  varieties  of  Cjuartzite  are  as  follows: 

Analysis  of  qiiaHzites  at  iase  of  Bkoabik  formation. 


1. 

2. 

3. 

4. 

SiO,                                              

18.26 
54.12 

None. 

None. 

76.96 

6.80 

.28 

None. 

77.82 

7.96 

.15 

.03 

84.21 

Fe                                            

2.42 

K,0                                      

p„0-                                      

p                                   

.015 

1.  Ferruginous  quartzite  at  base  of  iron  formation  (specimen  45668)  from  pits  southwest  of  sec.  3, 
T.  58  N.,  E.  17  W.     Analysis  by  E.  T.  Allen. 

2.  Green  quartzite  at  base  of  iron  formation   (specimen  45665)  from  pits  southwest  of  sec.  3, 
T.  58  N.,  K.  17  W.     Analysis  by  E.  T.  Allen. 

3.  Green  quartzite  at  base  of  iron  formation  (specimen  45655)  from  pits  in  southwest  of  sec.  3, 
T.  58  N.,  R.  17  AV.     Analysis  by  E.  T.  Allen. 

4.  Green  quartzite  near  base  of  iron  formation  (specimen  45687)  from  drill  hole  just  east  of  Fayal 
mine.     Analysis  by  R.  B.  Green. 

Above,  the  ferruginous  quartzite  becomes  irregularly  interbedded 
with  gnarled  and  contorted  phases  of  the  ferruginous  chert  described  on 
page  120.  Hand  specimens  were  collected  showing  several  interlaminated 
layers  of  the  normal  siliceous  ferruginous  chert  and  the  heavily  ferruginous 
quartzite.  The  two  kinds  of  rock  have  a  knife-edge  contact  (PI.  XII, 
fig.  B).  The  interbedded  quartzite  and  chert  are  but  a  few  feet  thick. 
Immediately  below  appears  conglomerate  a  foot  to  18  inches  in  thickness, 
resting  on  the  Pokegama  quartzite. 

In  the  east  end  of  the  range  is  another  phase  of  quartzitic  material 
basal  to  the  iron  formation.  This  is  essentially  a  heavily  ferruginous  chert 
characteristic  of  the  iron  formation,  but  containing  numerous  well-rounded 
quartz  grains  (specimens  46070  and  45119). 

The  basal  conglomerate  of  the  iron  f(irmation  is  usually  in  firm  contact 
with  the  underlying  massive  Pokegama  quartzite,  and  is  frequently  seen 


THE  BIWABIK  FORMATION.  157 

adhering  to  the  surface  of  snch  a  quartzite  after  the  softer  overlying  iron 
formation  has  been  worn  away  by  erosion.  It  sometimes  grades  into  the 
iron  formation  above  thi-otigh  the  quartzite  above  described,  and  sometimes 
grades  directly  into  the  iron  formation,  the  intermediate  quartzite  being 
lacking.  The  fragments  vary  in  size  from  that  of  sand  grains  to  6  inches 
in  diameter.  The  average  size  is  perhaps  an  inch  or  less.  The  fragments 
consist  of  vem  quartz,  of  quartzite  like  the  Pokegama  quartzite  immediately 
subjacent,  of  a  soft  yellow  limonitic  slate,  easily  eroded  and  frequently 
represented  in  part  by  cavities  in  the  conglomerate,  and  of  a  jaspery  phase 
of  the  ferruginous  chert  sometimes  approaching  iron  ore.  In  abundance  the 
fragments  stand  in  about  the  order  named.  Locally,  however,  the  jasper 
fragments  or  the  soft  yellow  slaty  fragments  may  be  more  abundant  than 
the  others.  The  matrix  is  almost  entirely  quartzite  in  various  degrees 
of  consolidation,  although  usually  fairly  hard.  In  some  of  the  associated 
rocks  in  which  the  jaspery  fragments  are  abundant  the  matrix  is  chert, 
containing  altered  greenalite  granules;  but  such  rocks  are  believed  to  be 
mainly  breccias,  belonging  with  the  gnarled  and  contorted  jaspery  basal 
phases  of  the  ferruginous  chert  (pp.  121-122). 

Both  the  nature  and  the  relative  abundance  of  the  pebbles  are  peculiar 
in  a  conglomerate  at  this  horizon.  One  would  expect  to  find  quartzite 
pebbles  in  greatest  abundance,  and  is  puzzled  by  the  presence  of  the  jas- 
pery chert  and  the  yellow  slaty  fragments.  Indeed,  in  the  early  part  of 
the  field  work  it  was  assumed  that  these  conglomerates,  containing,  as  they 
do,- phases  of  material  resembling  phases  of  the  iron  formation,  must  neces- 
sarily have  come  from  above  the  iron  formation,  and  it  was  not  until  they  were 
actually  observed  in  place  at  several  localities  that  it  could  be  believed  that 
they  represent  the  base  of  the  iron  formation.  The  scarcity  of  quartzite 
fragments  is  believed  to  be  due  to  the  breaking  up  of  the  sandstone,  the 
unindurated  equivalent  of  the  Pokegama  quartzite,  into  its  constituent  grains 
when  attacked  by  the  water  which  deposited  the  conglomerate.  The  uncon- 
formity between  the  conglomerate  and  the  quartzite  is  so  slight  that  it  is  cer- 
tain that  the  sand  now  represented  by  the  Pokegama  quartzite  was  only 
weakly  cemented  at  the  time  of  the  formation  of  the  conglomerate,  so  that 
when  attacked  by  the  waters  the  rock  was  disintegrated  into  its  constituent 
sand  grains,  and  only  rarely  a  large  fragment  remained.  It  has  been  noted 
til  at  the  abundant  mati'ix  of  the  conglomerate  is  composed  of  well-rounded 


158  THE  MESABI  IKON-BEAKING  DISTRICT. 

quartz  grains  of  remarkably  uniform  size.  Tliey  are  identical  in  size  and 
character  with  the  quartz  g-rains  of  the  underlving  Pokegama  formation, 
and  were  probably  derived  from  the  disintegration  of  the  sandstone. 

The  vellow  limouitic  slate  pebbles,  it  is  believed,  were  derived  from 
the  exceedingly  thin-bedded  or  fine  shaly  phases  of  the  underlying  Poke- 
gama  quartzite,  and  owe  their  yellow  and  soft  character  to  jjartial  replace- 
ment bv  iron  from  the  immediately  overlying  iron  formation.  In  the  8W.  ^ 
of  SW.  ^  sec.  3,  T.  58  N.,  R.  17  W.,  these  yellow  fragments  are  abundant 
in  the  conglomerate,  and  in  two  of  the  pits  close  at  hand  it  is  found  that 
the  phase  of  the  Pokegama  quartzite  immediately  underlving  is  a  fissile, 
almost  shaly  one,  and  that  some  of  it  has  been  altered  in  place  to  such  an 
extent  as  to  resemble  the  fragments  in  the  conglomerate.  It  is  apparent 
that  but  a  small  amount  of  further  alteration  would  be  necessary  in  this 
case  to  give  the  pebbles  the  characters  they  are  now  observed  to  have  in 
the  conglomerate.  '  ^ 

In  the  Mesabi  district,  the  jaspei-y  fragments  in  the  conglomerates  do 
not  have  their  counterparts  in  the  underlying  Pokegama  formation,  or  in 
the  Lower  Huroniau  or  Archean  rocks,  but  similar  rocks  are  known  in  the 
Lower  Huronian  and  Archean  rocks  of  other  iron-bearing  districts  of  Lake 
Superior,  and  may  have  been  exposed  in  the  past  even  in  the  Mesabi 
district,  having  been  removed  by  erosion  or  covered  up  by  later  formations. 
It  is  not  impossible  that  the  pebbles  may  have  been  carried  along  the  .shore 
for  long  distances.  The  jaspers  of  the  underlying  formations  in  adjacent 
districts  are  much  older  than  the  conglomerate,  and  must  have  been  almost 
as  hard  as  at  present  at  the  time  the  conglomerate  was  formed.  This  being 
the  case,  they  may  have  been  transported  for  many  miles  along  the  shore 
without  losing  their  integrity. 

While  the  explanation  of  the  nature  and  relative  abundance  of  the 
fragments  in  the  conglomerates  may  be  in  doubt  in  some  cases,  the  position 
and  relations  of  the  conglomerate  to  the  underlying  and  overlying  series 
are  certainly  known.  Enough  evidence  is  at  hand  to  warrant  the  statement 
that  the  base  of  the  iron  formation  is  conglomeratic  or  quartzitic,  or  both, 
throughout  tlie  district  and  that  these  frag-mental  rocks  var\'  in  thickness 
from  a  few  inches  to  15  or  more  feet.  Their  structural  significance  is 
discussed  in  the  section  on  relations  of  the  Biwabik  formation  to  other 
formations. 


THE  BIWABIK  FORMATION.  159 

Conglomerates  have  not  been  found  in  middle  or  upper  horizons  of 
the  Biwabik  formation,  except  at  one  place.  In  the  south  wall  of  the 
Mahoning  open  cut  a  layer  of  conglomerate  entirely  altered  to  iron  ore 
lies  between  layers  of  iron  ore  with  normal  texture. 

THE  ALTERATION    OF  THE   IRON    FORMATION   BY  THE   INTRUSION   OF 
KEWEENAWAN   GRANITE  AND   GABBRO. 

Through  ranges  12  and  13  the  iron  formation  is  intruded  on  the  north 
by  granite  and  on  the  south  by  the  Duluth  gabbro  (see  map,  PI.  II,  and 
description,  pp.  182-188),  and  has  undergone  considerable  metamorphism 
in  consequence.  This  metamorphism  has  extended  even  farther  west,  for, 
while  the  gabbro  does  not  come  into  actual  contact  with  the  iron  formation 
through  range  14,  it  abuts  against  the  overlying  Virginia  slate  and  has 
metamorphosed  both  the  slate  and  the  iron  formation  in  this  area." 

In  general  through  the  western  and  central  portions  of  the  Mesabi 
district  the  iron  oxide  of  the  iron  formation  is  mainly  hydrated  hematite, 
and  magnetite  is  in  subordinate  quantity.  Eastward  from  Mesaba  station 
the  iron  oxide  is  mainly  magnetite,  and  hematite  is  in  subordinate  quantity. 
Westward  from  Mountain  Ii'on  the  amphiboles  are  almost  entirely  lacking; 
from  Mountain  Iron  eastward  to  Mesaba  station  the  amphiboles  are  present 
in  the  iron  formation,  but  are  not  abundant  until  Mesaba  station  is 
approached;  eastward  from  Mesaba  station  they  become  abundant  and 
make  up  an  important  constituent  of  the  formation.  In  the  eastern  portion 
of  the  range  tlut  chert  is  correspouding-ly  less  abundant  than  in  the  western 
and  central  portions  of  the  district,  and  in  some  cases  is  almost  entirely 
absent.  The  chert  becomes  also  distinctly  coarser  in  this  area.  In  range 
12  the  grains  commonly  reach  a  diameter  of  3  or  4  millimeters,  and  there 
are  a  few  smaller  particles,  while  in  the  central  and  western  portions  of  the 
district  they  are  seldom  greater  than  0.10  millimeter,  and  almost  invari- 
ably are  associated  with  smaller  particles  (compare  Pis.  XV  and  XVII). 
Toward  the    east  there  is  a  tendency  for    the    texture    to    become  more 

"The  metamorphism  of  the  Biwabik  formation  by  the  gabbro  in  the  area  adjacent  to  Birch 
Lake  and  eastward  in  the  vicinity  of  Akeley  and  Gunflint  lakes  has  been  described  in  detail  by 
U.  S.  Grant  and  AV.  S.  Bayley,  and  has  been  briefly  considered  or  mentioned  by  N.  H.  Winchell, 
H.  V.  Winchell,  A.  H.  Elftman,  J.  E.  Spurr,  J.  ilorgan  Clements,  C.  R.  Van  Hise,  and  others,  as 
noted  in  the  review  of  literature  in  Chapter  II. 

The  intrusion  of  the  granite  and  its  metamorphic  effect  on  the  iron  formation  is  here  for  the  first 
time  described. 


160 


THE  MESABI  IRON-BEARING  DISTRICT. 


even,  although  there  are  many  wide  variations  from  uniformity.  The 
chert  grains,  instead  of  being  in  irregular,  roundish,  and  scalloped  cherty 
forms,  as  in  the  central  and  western  portions  of  the  district,  are  in  roughly 
polygonal  shapes  and  united  in  a  fairly  uniform  mosaic  (PI.  XVII).  Accom- 
panying these  changes  is  a  more  pronounced  segregation  of  the  magnetite 
and  the  amphibolitic  chert  into  irregular  layers  and  lenses,  with  the  result 
that  the  iron  oxide  layers,  instead  of  containing  various  other  minerals,  are 
comparatively  free  from  them.  The  characteristic  granules  of  the  ferrugi- 
nous cherts  are  still  conspicuous  in  the  east  end  of  the  district  but  in  the 
most  highly  metamorphosed  phases  of  the  rocks,  as  in  range  12,  they  have 


v:-  ■("'■'/■■ 


A 


N 


:-^.V.-l::\.>4 


Fig.  5. — Sketch  showing  relations  of  Embarrass  granite  to  the  Biwabik  formation  in  the  abandoned  glacial  gorge  in 

NW.  J  of  NW.  J  of  sec.  17,  T.  f.O  N.,  E.  12  \V. 

entirely  disappeared,  being  obscured  by  magnetite,  amphibole,  and  chert 
In  the  phases  not  showing  the  maximum  alteration  the}'  are  marked  by 
magnetite,  either  as  a  rim  about  the  granule,  as  a  solid  mass  filling  it,  or 
in  evenly  disseminated  particles  through  it.  Not  infrequently  the  granules 
maj^  be  observed  only  in  ordinary  light  and  then  by  distribution  of  the 
magnetitic  particles;  in  parallel  polarized  light  they  are  obscured  by 
the  polarization  of  the  amphibolitic  and  cherty  constituents.  Finally,  in  the 
eastern  portion  of  the  district  certain  minerals  have  developed  which  have 
not  been  found  in  the  less  altered  rocks  of  the  central  and  western  portions 


THE  BIWABIK  FORMATION. 


161 


of  the  Mesabi  district.  lu  the  latter  areas  the  ampliiboles  are  entirely 
gi-iinerite  and  actinolite,  with  little  or  no  hornblende.  In  the  eastern  por- 
tion of  the  district  the  amphiboles  include  griinerite  and  actinolite,  and  in 
addition  green  and  brown  hornblende  in  considerable  quantity.  Associated 
with  these  minerals  are  small  quantities  of  biotite,  glaucophane  (45057), 
andalusite  (45124),  zoisite  (45119),  and  g'arnet.  Still  farther  to  the  east,  in 
the  neighborhood  of  Gunflint  Lake,  the  continuation  of  the  Biwabik  forma- 
tion has  suffered  metamorphism  by  the  gabbro,  and,  according  to  Grant," 
there  has  been  an  extensive  development  of  hypersthene,  augite,  and  olivine, 
in  addition  to,   and  sometimes  replacing,   the  minerals  above  mentioned. 


Fig.  6. — Details  of  contact  of  Embarrass  granite  and  Biwabilj:  formation  in  the  gorge  in  N\V.  i  of  NW.  ^  of  sec.  17, 

T.  60  N.,  R.  12  W. 

While  hypersthene,  augite,  and  olivine  are  abundant  and  characteristic  in 
the  true  gabbro  of  range  12  and  westward,  these  minerals  are  nearly,  if 
not  quite,  lacking  in  the  Biwabik  formation. 

Although  eastward  toward  Gunflint  Lake  the  gabbro  alone  has  been 
able  to  produce  even  greater  metamorphic  effects  on  the  iron-bearing  rocks, 
it  is  certain  that  the  metamorphism  of  the  iron-bearing  rocks  in  the  region 
under  description  has  been  produced  jointly  by  the  gabbro  and  granite. 


«  See  Chapter  II. 


MON    XLIII  — 03- 


-11 


162  THE  MESABI  IRON-BEARING  DISTRICT. 

At  the  contacts  of  the  iron  formation  with  each  of  tliese  rocks,  detailed 
evidence  of  their  metamorphic  effects  may  be  observed.  For  the  gabbro 
contact  the  general  description  of  the  metamorphism  above  given  applies 
almost  in  toto,  the  only  exception  being  the  statements  concerning  the 
augite,  biotite,  and  andalusite,  which  are  not  found  near  the  gabbro  contact. 

The  granites,  before  this  work  was  taken  up,  had  not  been  known  to 
be  intrusive  in  the  iron  formation,  and  hence  a  brief  detailed  description 
of  its  contact  effects  may  be  of  interest. 

The  contact  effects  of  the  granite  may  be  well  studied  in  the  gorge  in 
the  NW.  i  of  NW-  i  of  sec.  17,  T.  60  N.,  R.  12  W.,  where  on  both  sides 
of  the  gorge  are  actual  contacts  of  the  rocks.  (See  figs.  5  and  6,  and 
description  of  structural  relations,  pp.  187-1 88.)  The  iron  formation  is  a  very 
dark-gray  and  grayish-yellow  ferruginous  chert,  highly  siliceous,  containing 
near  its  base  a  few  undoubted  well-rounded,  fragmental  quartz  grains.  The 
chert  does  not  contain  iron  oxide  in  bands  but  in  evenly  disseminated 
particles  and  aggregates,  in  some  layers  in  so  small  a  quantity  as  to  make 
the  chert  resemble  a  quartzite  in  general  aspect.  The  only  structure  resem- 
bling a  banding  is  a  rough  parting  or  jointing  into  thick,  nearly  horizontal, 
layers.  The  massive  character  of  the  rock  here  distinguishes  it  from  the 
well-banded  phases  away  from  the  contact. 

At  the  to])  of  the  gorge,  and  down  to  within  a  few  feet  of  the  contact, 
the  iron-formation  rocks  look  siliceous  and  have  disintegrated,  brown, 
sugary  surfaces  where  weathered.  Under  the  microscope  the  quartz  appears 
in  medium-sized  grains  which  have  a  tendency  to  be  of  uniform  size 
for  certain  bands,  although  varying  somewhat  in  different  bands.  The 
quartz  grains  are  of  polygonal  shape  and  are  fitted  together  in  a  fairly 
uniform  mosaic-  No  trace  of  clastic  structure  is  to  be  observed,  unless 
aggregation  into  indefinite  bands  be  taken  as  evidence  of  sedimentary  ori- 
gin. Evenly  disseminated  through  the  quartzose  background  are  particles 
and  irregular  ao-OT-egfates  of  iron  oxide,  mainlv  niao:netite,  of  o-reeu  and 
brown  hornblende,  of  actinolite  or  griiuerite,  of  zoisite,  and  of  biotite.  In 
abundance  the  minerals  stand  in  about  the  order  named.  The  actinolite 
and  griiuerite  are  in  niinute,  isolated,  columnar  forms  and  in  radial  and 
sheaf-like  aggregates. 


THE  BIWABIK  FORMATION.  163 

Within  a  few  feet  of  the  contact  the  ferrug-inous  chert  shows  a  black, 


heavih^  feiTUginous  background,  in  which  beautifully  rounded  quartz  eyes 
are  highly  conspicuous  because  of  their  translucent  character  and  their 
dark  reflections.  Under  the  microscope  the  grains  appear  well  rounded 
and  in  some  cases  even  show  incipient  enlargement.  The  matrix  consists 
of  greenish,  transparent  zoisite  plates  and  dark,  greenish-black  aggregates 
of  magnetite,  zoisite,  hornblende,  griinerite,  and  actinolite  (45119).  The 
forms  of  the  ellipsoidal  granules  are  still  to  be  observed  in  ordinary  light 
by  distribution  of  the  iron  oxide.  Under  crossed  nicols  the  other  minerals 
named  completely  obscure  them.  It  is  clear  in  connection  with  the  altera- 
tions traced  in  the  central  part  of  the  district  that  the  rock  was  originally 
composed  of  ferrous  silicate  granules  and  of  fragmental  quartz  grains  in 
about  equal  abundance,  and  that  the  alteration  of  the  ferrous  silicate  has 
given  the  dense,  dark-green  and  black  background  in  which  the  quartz 
grains  now  stand  so  conspicuously. 

Immediately  next  to  the  contact  with  the  granite,  the  quartz  in  the 
matrix  of  the  ferruginous  chert  has  been  thoroughly  recrystallized,  consid- 
erably' increased  in  size,  and  the  round  shape  lost.  The  granite  is  rich  in 
quartz  which  occurs  as  purple  phenocrysts.  Within  a  few  inches  of  the 
contact  almost  identical  quartz  phenocrysts  have  been  developed  in  the 
iron  formation  (see  fig.  A,  PL  XVIII),  and  in  addition  numerous  irregular 
stringers  and  veins  of  quartz  ramify  through  the  iron  formation.  It  looks 
as  if  there  had  been  a  minute  injection  of  the  quartz  into  the  iron  formation 
close  to  the  contact  through  the  agency  of  hot  silica-bearing-  solutions 
accompanying  the  intrusion  of  the  granite.  Several  specimens  were 
collected,  which  consist  of  about  one-half  quartz  in  stiingers  and 
phenocrysts  and  about  one-half  iron-formation  material.  The  contact  of 
the  iron  formation  and  granite  is  ordinarily  exceedingly  sharp  (see  fig.  A,  PI. 
XVIII)  though  in  places  an  irregular  gradation  zone  from  an  eighth  of  an 
inch  to  an  inch  thick  separates  the  two.  In  this  zone  may  be  also  obsei'ved 
jjink  feldspars  and  purplish  quartz  characteristic  of  the  granite,  lying  in  a 
black,  ferruginous  chert  matrix,  which,  toward  the  granite  side,  fades  out 
and  toward  the  iron-formation  side  becomes  more  abundant  until  it  excludes 
the  feldspar  of  the  granite.  In  this  zone  andalusite  (45124)  and  biotite 
(45124),  commonly  associated,  and  glaucophane  have  been  developed  in 
small  quantity,  and  the  quantity  of  green  hornblende,  actinolite,  and  griinerite 
has  been  increased. 


164  THE  MESABI  IRON-BEARING  DISTRICT. 

COMPARISON  OF  THE   METAMORPHIC  EFFECTS  OF  THE  GRANITE  AND   GABBRO. 

The  contact  effects  produced  by  the  granite  differ  from  those  produced 
b}'  the  gabbro  in  the  following  ways : 

(1)  While  there  has  been  thorough  recrystallization  next  to  the  granite 
contact,  the  size  of  the  grains  has  not  increased  nearly  so  much  as  next  to 
the  gabbro  contact. 

(2)  Accompanying  the  recrystallization  next  to  the  gabbro  there  has 
been  a  tendency  toward  the  segregation  of  the  magnetite  into  irregular 
masses  and  layers,  it  is  possible  to  take  out  good-sized  hand  specimens  of 
almost  clear  magnetite.  Next  to  the  granite  contact  it  would  be  difficult 
to  find  magnetite  well  enough  segregated  to  allow  of  this. 

(3)  Next  to  the  granite  contact  there  have  been  developed  andalusite, 
zoisite,  biotite,  and  glaucophane  which  are  not  characteristic  of  the  gabbro 
contact  in  the  area  west  of  Birch  Lake.  On  the  other  hand,  green  and 
brown  hornblende  are  much  less  abundant  than  near  the  gabbro  contact, 
and  olivine  and  h^'persthene,  which  in  the  Gunflint  Lake  area  are  charac- 
teristic of  the  gabbro  contact  (although  not  observed  west  of  Birch  Lake), 
are  altogether  lacking  near  the  granite. 

(4)  Tlie  intrusion  of  the  granite  has  caused  the  introduction  of  a 
considerable  amount  of  quartz  in  stringers  and  phenocrysts  into  the  iron 
formation  adjacent  to  the  contact.  The  gabbro  has  contributed  little  or 
no  material  to  the  iron  formation  in  tlie  contacts  observed  in  the  Mesabi 
district.  To  the  east,  near  Akeley  and  Gunflint  lakes,  such  transfer  from 
the  gabbro  probably  occurred." 

MAGNETIC    ATTRACTION. 

The  normal  magnetic  variation  for  northeastern  ]\Iinnesota  where  there 
is  no  local  disturbance  is  7°  east  of  north.  Throughout  the  Biwabik 
formation  the  needle  shows  deflections  from  this  direction.  In  limited  areas 
the  deflections  of  the  needle  are  most  capricious,  yet  by  putting  together 
observations  taken  throughout  the  range  it  becomes  apparent  that  there  is 
some  regularity  in  the  magnetic  attraction.  In  the  eastern  portion  of  the 
district  a  high  and  variable  deflection  can  be  counted  upon,  for  here,  as  we 
have  seen,  there  is  much  magnetite,  due  to  the  alteration  of  the  Keweenawan 

«See  disi.'Ussion  of  (labbro  contact  by  Grant,  Bayley,  ami  utluTs  sniiiiiiarizod  in  Cliajiter  11. 


THE  BIWABIK  FORMATION.  165 

intrusives,  and  the  drift  covering  is  thin.  Variations  of  40°  or  50°  or  even 
90°  are  common.  Throng-h  the  central  and  western  portions  of  the  district 
the  magnetic  needle,  on  an  average,  varies  but  a  few  degrees  from  the 
normal,  but  so  far  as  there  is  any  variation  it  is  greater  in  certain  zones 
than  in  others.  One  of  these  is  just  within  the  iron  formation  near  its 
contact  with  the  Pokegama  quartzite.  It  has  a  width  varying  from  a  few 
steps  to  a  few  hundred  steps.  In  this  zone  variations  as  high  as  30°  to 
40°  are  common.  From  this  zone  tongues  of  liigh  attraction  project  to 
the  south.  Also  in  the  neighborhood  of  some  of  the  ore  deposits  a  slightly 
higher  attraction  has  been  noted  near  the  contact  of  the  ore  with  the  wall 
rock  than  in  the  rock  or  in  the  ore. 

STRUCTURE. 

The  most  conspicuous  structure  in  the  Biwabik  formation  is  the 
bedding,  which  may  be  observed  in  all  pliases  of  the  formation.  The 
bedding  layers  may  be  several  feet  thick  or  as  thin  as  those  of  shales.  The 
more  massive  and  irregular  layers  and  the  poorest  parting  are  shown  by 
the  ferruginous  chert,  particularly  the  feiTUginous  chert  in  lower  horizons 
of  the  formation.  The  finer  bedding,  accompanied  by  a  better  parting,  is 
shown  by  the  iron  ores,  by  the  slaty  rocks  within  the  iron  formation,  and 
by  some  of  the  ferruginous  cherts  in  middle  and  upper  horizons.  The 
iron  formation  is  a  sedimentary  formation  interbedded  with  typical  quartzite 
and  slate,  and  the  bedding  as  a  whole  is  an  original  structure,  but  the 
formation  has  undergone  such  great  metamorphism  that  the  bedding-  has 
suffered  many  modifications.  The  iron  formation  originally  consisted 
mainly  of  greenalite  rocks  with  thin  interbanded  layers  of  slate.  The 
alteration  of  the  greenalite  rocks  to  ferruginous  chert  has  been  accom- 
panied by  the  segregation  of  the  iron  and  silica  into  irregular  bands  and 
lenses  which  serve  to  mark  the  position  of  the  original  bedding,  but  at  the 
same  time  obliterate  much  of  it  and  make  the  parting  parallel  to  the 
bedding  very  poor.  The  alteration  of  the  greenalite  rocks  to  the  ore 
deposits  has  not  obliterated  any  of  the  original  bedding,  but  on  the  contrary 
has  made  it  even  more  conspicuous  and  has  made  the  parting  parallel  to  it 
an  excellent  one. 

The  iron  formation  has  been  tilted  gently  away  from  the  high  land 
of  the  Giants  range  adjacent  at  an  angle  varying  from  5°  to  20°,  but 


166  THE  MESABI  IRON-BEARING  DISTRICT. 

averaging'  perhaps  8°  or  10°,  and  has,  in  addition,  been  gently  flexed 
both  parallel  and  transverse  to  the  range.  All  of  these  features  are 
indicated  by  the  variation  in  direction  and  degree  of  dip  in  different  parts 
of  the  formation,  and  may  be  actually  observed  on  a  small  scale  in  the 
open  cuts  of  the  mines.  Accompanying  the  folding  is  fracturing  and  even 
brecciation.  The  formation  is  cut  by  numerous  joints,  which  in  the 
massive  cherty  portions  are  even  and  continuous,  but  which  in  the  fine 
bedded  portions,  and  particularly  in  the  ore  deposits,  are  extremely 
irregular  and  for  the  most  part  coterminous  with  the  individual  layers. 
In  some  places,  as  at  the  Biwabik  mine,  and  at  the  Hale  and  Kanawha 
mines,  there  is  evidence  that  the  faulting-  and  brecciation  of  the  iron 
formation  has  been  greater  near  the  contact  of  the  iron  formation  with  the 
underlving  rocks  than  farther  away  from  it.  This  is  due  to  readjustment 
along  the  contact  during  the  folding  of  the  district. 

Water  flows  easily  through  the  formation  parallel  either  to  the 
bedding  or  to  the  joints.  In  the  massive  portions  it  probably  is  more 
nearly  parallel  to  the  joints  than  to  the  bedding,  because  of  the  poorer 
parting  parallel  to  the  latter.  But  in  the  more  finely  bedded  portions  the 
water  could  move  in  almost  any  direction. 

Most  of  these  structural  features  the  iron  formation  shares  with  the  other 
members  of  the  Upper  Hurouian  series,  and  are  discussed  more  in  detail  in 
connection  with  the  structure  of  the  Upper  Huronian  series  (pp.  178-1 8U). 

THICKNESS. 

The  thickness  of  the  Biwabik  formation  has  been  directly  measured  in 
but  one  place,  in  sec.  34,  T.  59  N.,  R.  14  W.,  where  Mr.  E.  J.  Longyear 
drilled  from  the  Virginia  slate  through  the  iron  formation  into  greenstone 
below,  and  found  the  iron  formation  to  have  a  thickness  of  576  feet.  On 
the  basis  of  average  dips  and  width  of  exposure  of  the  iron  formation  at 
any  one  place  the  thickness  might  be  more  than  2,000  feet  or  as  little  as 
200  feet.  Sucli  great  variation  is  in  part  real  and  in  part  apparent.  The 
iron  formation  grades  laterally  into  the  Virginia  slate  (see  pp.  172-176)  and 
the  fi)rmation  was  dejiosited  against  an  irregular  shore  line  and  an  irregular 
bottom  of  the  Lower  Huronian  and  Archean  rocks,  both  causing  variation 
in  the  real  thickness  of  the  formation.  On  the  other  Imnd  the  gentle 
foldings  of  tlic  formation  in  two  directions,  cou])led  witli  the  scarcity  of  dip 


THE  BIWABIK  FORMATION.  167 

observations,  make  the  selection  of  a  fair  average  dip  across  the  formation 
at  any  given  place  an  exceedingly  difficult  matter,  and  some  of  the  apparent 
variation  in  thickness  is  unquestionably  due  to  failure  to  take  into  account 
the  minor  folding.  The  average  thickness  for  the  district  is  perhaps  in  the 
neighborhood  of  1,000  feet.  The  eastward  continuation  of  the  iron  forma- 
tion, near  Gunflint  Lake,  has  been  estimated  by  Grant"  to  have  a  thickness 
of  825  feet. 

RELATIONS  TO  OTHER   FORMATIONS. 

The  Pokegama  quartzite  underlies  the  Biwabik  formation  for  most  of  the 
district.  The  relations  are  those  of  a  minor  erosion  interval.  Structurallv 
the  two  formations  conform  to  each  other;  so  far  as  can  be  ascertained,  the 
dips  of  the  Pokegama  quartzite  are  identical  with  those  of  the  overlying 
iron  formation.  The  Pokegama  quartzite  had  not  been  folded  prior  to  the 
deposition  of  the  iron  formation,  and,  as  members  of  the  Upper  Huronian 
series,  both  formations  have  been  affected  by  the  gentle  Upper  Huronian 
folding.  However,  at  a  considerable  number  of  places  in  the  district,  and, 
it  is  believed,  throughout  the  district,  a  layer  of  conglomerate  a  few  inches 
to  several  feet  in  thickness  separates  the  iron  formation  from  the  Pokegama 
quartzite.  The  pebbles  in  this  conglomei'ate  are  in  small  part  of  quartzitic 
rocks  like  those  underlying  and  show  some  variety,  and  hence  the  con- 
glomerate must  indicate  .the  lapse  of  some  time  interval  prior  to  its 
deposition.  Yet,  taken  in  connection  with  the  thinness  of  the  conglom- 
erate, the  characteristic  scarcity  of  indurated  quartzite  fragments  (see  pp. 
157-158),  the  comparatively  small  size  of  its  fragments,  and  the  corre- 
spondence in  dip  of  the  quartzite  to  the  iron  forination,  it  seems  likely  that 
the  erosion  interval  represented  by  the  conglomerate  was  not  a  g-reat  one. 
The  conglomerate  is  closely  associated  with  a  brecciated  phase  of  the 
ferruginous  chert,  and  in  some  places  it  is  hard  to  draw  a  line  between 
the  two.  The  breccia  indicates  that  in  addition  to  the  normal  erosion 
unconformity  there  has  been  a  slig'ht  amount  of  readjustment  along  the 
jjlane  of  contact  between  the  quartzite  and  the  iron  formation,  .which 
probably  occurred  during  the  tilting-  and  folding  of  the  Upper  Huronian 
strata.  The  Pokegama  quartzite  and  the  iron  formation  pr,.,bably  acted 
essentially  as  units  during  the  deformation. 

«  Geol.  Nat.  Hist.  Survey  Minnesota,  Final  Eeport,  Vol.  IV,  1899,  p.  486. 


168  THE  MESABI  IRON-BEARING  DISTRICT. 

For  short  intervals  in  the  district  the  Pokegama  qnartzite  has  not 
been  found  between  the  Biwabik  formation  and  the  underlying  rocks,  and 
in  such  areas  the  iron  formation  rests  unconformably  on  the  Archean  and 
Lower  H'uronian  rocks. 

Through  much  of  ranges  12  and  13  the  Biwabik  formation  lies 
directly  upon  intrusive  granite,  probably  of  Keweenawan  age. 

The  Biwabik  formation  is  o^•erlain  for  the  most  of  the  district  by 
the  Virginia  slate.  In  the  eastern  portion  of  the  district  evidence  is  not 
at  hand  to  show  the  distribution  of  the  Virginia  slate,  but  jJi'esumably  it 
is  the  rock  immediately  south  and  overlapping  the  iron  formation  until 
cut  out  by  the  overlapping  of  the  Keweenawan  gabbro.  The  Virginia 
slate  and  the  Biwabik  formation  are  perfectly  conformable.  The  Biwabik 
formation  throughout  contains  layers  of  slate  which,  near  the  upper 
horizons,  become  more  numerous  and  finally  form  the  Virginia  slate. 
Moreover,  the  upper  part  of  the  iron  formation  grades  laterally  into 
slate,  which  has  been  mapped  as  the  Virginia  formation.  The  relations 
of  the  Virginia  slate  to  the  Biwabik  formation  are  more  fully  discussed  in 
connection  with  the  Virginia  slate. 

Throuffh  much  of  rang-es  12  and  13  the  iron  formation  is  overlain 
directly  by  the  gabbro,  and  on  the  north  side  of  Birch  Lake  the  north- 
easternmost  tongue  of  the  iron  formation  is  bounded  both  east  and  west 
by  the  gabbro. 

SECTIOjS^  III.     VIRGIIVIA  SLATE. 

DISTRIBUTION. 

The  Virginia  slate  bounds  the  iron  formation  on  the  south  from  the 
west  end  of  the  district  to  near  the  east  side  of  sees.  5  and  8,  R.  13  W., 
where  the  slate  is  overlapped  by  the  gabbro.  Still  farther  east  in  the  SW. 
J  sec.  25,  T.  60  N.,  R.  13  W.,  drilling  has  shown  altered  slate  to  lie  between 
Keweenawan  gabbro  on  the  south  and  Keweenawan  diabase  on  the  north, 
but  whether  it  is  an  isolated  mass  at  this  point  in  the  Keweenawan  area,  or 
is  continuous  with  the  slate  to  the  west,  explorations  or  exposures  do  not 
yet  tell.  The  slate  underlies  the  lower  slopes  of  the  Mesabi  range  and  con- 
tinues soutli  under  the  low-lying  swampy  area  south  of  the  Mesabi  range 
for  an  unknown  distance.  The  area  overlain  by  slate  is  so  thickly  covered 
with  drift  tlint  exposures  of  the  slate  are  almost  entirely  lacking;  its  pres- 
ence and  distribution  have  been   determined   bj'  drilling  and  test  pitting 


THE  VIRGINIA  SLATE.  169 

done  in  the  search  for  iron.  Through  the  central  portion  of  the  district 
enough  of  such  work  has  been  done  to  show  the  position  of  the  slate  bound- 
ary with  a  fair  degree  of  accuracy,  although  even  here  there  are  consider- 
able stretches  where  records  of  the  occui'rence  of  slate  are  wanting.  In  the 
western  and  eastern  portions  of  the  district  the  distribution  of  the  slate  is 
largely  hypothetical,  particularly  so  in  the  western  end  of  the  district.  In 
drawing  the  slate  line  on  the  map  of  this  portion  of  the  area,  all  that  can  be 
done  is  to  connect  the  widely  separated  explorations  which  reveal  slate. 
Wherever  exploration  has  been  detailed  it  is  found  that  the  slate  boundary 
is  not  straight,  but  in  gentle  curves,  and  it  is  reasonable  to  expect,  therefore, 
that  future  work  will  show  numerous  additional  undulations  in  the  slate 
boundary  for  the  area  at  present  not  completely  explored. 

KINDS   OF   ROCKS. 
SLATE. 

The  normal  Virginia  slate  is  usually  a  gray  rock,  though  in  part  black, 
reddish,  or  brown,  with  bedding  shown  by  alternating  bands  of  varying 
color  and  texture.  Some  of  the  beds  are  almost  coarse  enough  to  be  called 
graywackes.  Indeed,  in  the  field  the  rock  has  been  called  a  banded  slate 
and  gray  wacke.  Some  of  the  slate  is  hard  and  siliceous,  while  other  phases, 
especially  the  nonsiliceous  and  carbonaceous  ones,  are  soft  and  can  be  whit- 
tled with  a  knife.  Near  the  contact  of  the  slate  with  the  iron  deposit  in  the 
underlying  ii'on  formation,  as  at  Biwabik  and  in  sec.  3,  T.  58  N.,  R.  15  W., 
the  slate  becomes  iron  stained  and  soft  and  grades  into  paint  rock.  The  slate 
in  general  has  a  very  poor  parting  parallel  to  bedding  planes,  and  there  is 
little  or  no  development  of  secondary  cleavage.  What  there  is  of  secondary 
cleavage  has  been  developed  parallel  to  the  bedding  planes  and  is  marked 
by  minute  particles  of  mica  there  found.  While  the  rock  in  general  aspect 
and  minei'alogic  and  chemical  composition  looks  like  slate,  it  differs  from  a 
true  slate  in  lacking  a  true  cleavage,  and  as  this  is  one  of  the  essential 
characteristics  of  slate,  it  may  be  doubtful  whether  the  term  slate  ought 
to  be  applied  to  the  rock.  Yet  the  rock  is  not  a  shale,  for  it  is  too 
much  metamorphosed  and  has  too  poor  a  parting  parallel  to  the  bedding-. 
In  the  eastward  continuation  of  the  Mesabi  district,  east  of  Gunflint 
Lake,  the  same  formation  shows  the  characteristics  of  a  true  slate,  and  the 
formation  both  here  and  in  the  Mesabi  district  proper  has  been  known 


1  70  THE  MESABI  IRON-BEARING  DISTRICT. 

locally  and  in  geologic  literature  as  slate.  Hence  the  term  is  here 
retained. 

Under  the  microscope  the  rock  is  seen  to  be  a  chloritic  slate  containing 
minute,  more  or  less  angular,  pieces  of  quartz,  and  perhaps  some  of  feldspar, 
associated  with  chlorite  in  confused  aggregates.  The  chlorite  makes  up 
about  half  the  rock.  An  occasional  minute  plate  of  mica  is  present  with 
its  greater  diameter  parallel  to  the  bedding,  possibly  as  a  result  of  the 
incipient  development  of  secondary  cleavage.  In  general  the  grain,  while 
exceedingly  tine  for  some  layers,  is  fairly  coarse  for  a  slate.  The  carbo- 
naceous slates  differ  from  the  ordinary  type  only  iu  containing  minute 
specks  of  carbon  tending  to  obscure  the  other  constituents. 

A  partial  analysis  of  the  slate  (sjDecimen  4.o735),  from  south  of  the 
"Meadow"  explorations  in  sec.  3,  T.  58  N.,  R.  15  W.,  by  H.  N.  Stokes,  ot 
the  United  States  Geological  Survey,  is  as  follows: 

Analysis  of  Yirginia  slate. 

SiOj , 46. 61 

AljOa  8. 04 

FejOs 25.  77 

Fe  .  - 18.  04 

MgO 2.  82 

CaO 13 

PA 06 

CO, None. 

The  following  analysis  by  Mr.  George  Steiger,  of  the  United  States 
Geological  Survey,  is  of  a  composite  sample  of  the  Virginia  slate  made  up 
b}'  assembling  seyeral  specimens  from  two  localities  (specimen  45767  from 
excavation  for  watei*  tank  of  Eastern  Railwav  of  Minnesota  at  Virofinia; 
specimen  45463  from  south  of  the  Biwabik  mine): 

SiOj 62.  26 

AlA ; 16.  89 

FeA - 1-76 

FeO 4.  55 

MgO 2. 95 

CaO 42 

Na,0 2.  29 

K,6 :^.02 

HjO- 0.  70 

HjO-l- 3.  88 

TiO, 60 

CO, None. 

P,(i, 20 

Organic  inuk'tcrniintMl 

99.  52 


THE  VIRGINIA  SLATE.  171 

I 

LIMESTONE. 

In  the  lower  horizons  of  the  Virginia  slate,  as  well  as  in  the  upper 
horizons  of  the  iron  formation,  is  a  considerable  amount  of  limestone, 
interbedded  with  both  slate  and  the  iron"  formation.  The  rock  effervesces 
readily  with  cold  dilute  hydrochloric  acid,  and  hence  probably  does  not 
contain  much  magnesia.  The  layers  vary  in  thickness  from  several  feet  to 
a  fraction  of  an  inch.  Near  the  horizon  of  the  rapid  transition  of  the  slate 
into  the  iron  formation  there  is  a  fairly  persistent  layer  of  the  limestone 
several  feet  thick,  although  at  many  places  it  is  lacking  at  this  horizon. 
This  may  be  well  studied  in  the  excavation  at  the  water  tank  of  the  Eastern 
Railway  of  Minnesota  at  Virginia.  The  limestones  are  gray  rocks,  not  to 
be  discriminated  by  their  color  or  texture  from  some  of  the  coarser  phases 
of  the  banded  slates.  Indeed,  in  some  cases  it  was  not  until  a  test  had  been 
made  with  hydrochloric  acid  that  the  presence  of  carbonate  was  suspected. 
(Specimen  45464.)  Under  the  microscope  the  calcite  is  observed  in  irreg- 
ular grains  and  aggregates  making  up  the  bulk  of  the  rock.  Associated 
with  the  calcite  are  small  quantities  of  quartz  and  chlorite  in  particles  of 
about  the  same  shape  and  size  as  those  seen  in  the  slate  above  described. 

COKDIEEITE-HORNSTONE    RESULTING    FROM   THE    ALTERATION    OF    THE    VIRGINIA    SLATE 

BY   THE    GABBEO. 

In  approaching  the  gabbro,  which  overlaps  the  Virginia  slate  in  ranges 
14  and  13,  the  slate  becomes  more  crystalline,  harder,  and  characteristically 
breaks  with  a  conchoidal  fracture,  and  the  color  becomes  darker  and 
frequently  is  a  bluish  black.  The  rock,  indeed,  becomes  a  hornstone. 
Moreover,  there  appear  minute,  light-colored  specks  which  on  the  weathered 
surface  are  likely  to  have  disappeared  and  to  be  represented  by  pits. 
Under  the  microscope  the  white  specks  are  found  to  be  cordierite  in  typical 
development,  standing  as  numerous  phenocrysts  in  a  fine-grained  matrix  of 
biotite,  feldspar,  magnetite,  and  certain  doubtful  microlites  which  may  be 
actinohte  or  sillimauite,  or  both."  (Figs.  B  and  C  of  PI.  XIX.)  The 
cordierite  ciystals  appear  in  short,  columnar  forms  or  with  hexagonal 
outlines,  depending  upon  their  position  with  reference  to  the  plane  of  the 
slide.     Their  average  diameter  is  between  0.10  and  0.20  millimeter,  and 

"  Cordierite  in  this  formation  was  first  noted  and  described  by  N.  H.  Winchell,  Geol.  Nat.  Hist. 
Survey  Minnesota,  Final  Kept.,  Vol.  V,  1900. 


172  THE  MESABI  IRON-BEARING  DISTRICT. 

their  length  varies  from  0.40  to  0.80  millimeter.  The  single  and  double 
refractions  are  low  and  striking-ly  similar  to  those  of  quartz.  Their  dis- 
tinguishing feature  is  the  twinning,  which  in  basal  or  hexagonal  sections 
causes  the  crystals  to  lighten  and  darken  in  alternating  sextants  in  revolv- 
ing under  crossed  nicols.  The  cordierite  crystals  are  more  or  less  obscured 
by  minute  blades  of  biotite,  and  perhaps  other  micas,  and  by  nmnerous 
columnar  raicrolites  of  some  greenish-white  mineral,  without  pleoclii'oism 
and  with  high  single  and  low  double  refraction,  whose  character  because  of 
their  small  size  is  doubtful.  They  may  be  sillimanite,  one  of  the  chai'acter- 
istic  associates  of  cordierite.  Both  macroscopically  and  microscopically  the 
metamorphosed  slate  is  cordierite-Jiornstone,  as  this  term  is  used  by  petrog- 
raphers  such  as  Rosenbusch,  Zirkel,  and  Harker. 

A  partial  analysis  of  the  cordierite  hornstone  (specimen  45699),  from 
near  the  southeast  corner  of  the  NE.  J  of  SE.  J  sec.  21,  T.  59  N.,  R.  14  W., 
by  H.  N.  Stokes,  of  the  United  States  Geological  Survey,  is  as  follows: 

Analysis  of  cordierite-h/)rnsto7ie. 

SiOj 70. 48 

AljOj 12. 46 

FejOj 4.  94 

MgO 2. 04 

CaC) 71 

V.,0; 11 

Cbj None. 

From  the  Crystal  Falls  district,  south  of  Lake  Superior,  Clements"  has 
described  and  figured  a  spilosite  which  shows  a  most  remarkable  similarity 
to  the  cordierite- hornstone  here  described  The  j^orphyritic  crystals  and 
the  background  have  the  same  general  aspect,  but  when  examined  closely 
the  phenocrysts  in  the  rock  described  hj  Clements  are.  found  to  be  albite 
instead  of  cordierite. 

RELATIONS   OF  THE  VIRGINIA  SLATE  TO  THE  BIWABIK  FORMATION. 

Reference  has  already  been  made  to  the  fact  that  the  relations  of  the 
Virginia  slate  to  the  underlying  Biwabik  formation  are  those  of  gradation, 
both  lateral  and  vertical.  It  remains  to  discuss  this  gradation  somewhat 
fully.  The  iron  formation  contains  slate  layers  throughout.  In  upper 
and  middle  horizons  they  are  perhaps  more  numerous  than  in  lower  hori- 

«The  Crystal  Falls  iron-bearing  district  of  Michigan,  by  J.  Morgan  Clements:  Men.  U.  S.  Geol. 
Survey  Vol.  XXXVI,  IsiW,  pp.  206-207,  PI.  XXXVI. 


PLATE   XIX 


173 


PLATEXIX. 

PHOTOMICROGRAPHS   SHOWING    METAMORPHISM    OF    VIRGINIA    SLATE    INTO    CORDIEKITE- 
HORNSTONE   IN    APPROACHING    DULUTH    (gREAT)    GABBRO. 

Fig.  a. — Slate.  Specimen  45463,  slide  15736.  From  water  tank  of  Eastern  Railway  of  Minnesota, 
Virginia.  With  analyzer,  x  70.  This  is  the  normal  flne-grained  Virginia  slate,  consisting  of  quartz 
and  feldspar,  chlorite,  muscovite,  biotite,  and  iron  oxide,  all  of  the  constituents  arranged  with  their 
longer  diameters  roughly  parallel,  though  the  chlorite  and  iron  oxide  are  arranged  to  a  less  extent 
than  the  other  constituents.     Described,  pp.  169-170. 

Fig.  B. — Same,  without  analyzer. 

Fig.  C.  — Cordierite-hornstone.  Specimen  45285,  slide  15679.  From  near  east  quarter  post  of 
sec.  27,  T.  59  N.,  R.  14  W.  Without  analyzer,  x  70.  This  rock  results  from  the  alteration  of  the 
normal  Virginia  slate  by  the  contact  of  the  Duluth  gabljro.  The  light  round  and  oval  spots  are 
cordierite,  which  in  ordinary  light  can  scarcely  be  distinguished  from  quartz  or  feldspar.  The  matrix 
is  the  flne-grained  one  normal  to  the  slate.     Described,  pp.  171-172. 

Fig.  D. — The  same,  with  analyzer,  x  70.  The  cordierite  crystals  may  be  distinguished  by  the 
trillings  faintly  shown  on  the  round  basal  sections.  As  in  ordinary  light  the  cordierite  resembles 
albite  and  quartz.  In  general  aspect  the  cordierite-hornstone  here  described  corresponds  almost  exactly 
to  the  spilosite  of  the  Crystal  Falls  district  described  by  Clements  and  to  the  feldspathic  graywacke  of 
the  Penokee-Gogebic  district,  described  by  Van  Hise.  In  both  of  these  cases,  however,  the  phenocrysts 
are  albite  instead  of  cordierite.     Described,  pp.  171-172. 

174 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH  XLIll   PL.  XIX 


PHOTOMICROGRAPHS  SHOWING   METAMORPHISM  OF  VIRGINIA  SLATE  INTO  CORDIERITE 
HORNFELS  IN  APPROACHING  GREAT  GABBRO  FORMATION. 


HE   MERIDEN   GRAVURE   CO. 


THE  VIRGINIA  SLATE.  175 

zons.  Just  below  the  solid,  black  Virgiuia  slate  there  is  a  zone  in  which 
there  are  many  interlaminations  of  iron  formation  and  slate,  the  layers 
varying  in  thickness  from  several  feet  to  a  fraction  of  an  inch.  This  zone 
is  of  varying  and  uncertain  thickness.  In  many  places  at  least  the  zone  of 
minute  interbanding  is  thin,  not  more  than  15  or  20  feet,  but,  as  already 
noted,  layers  of  slate  are  found  well  down  in  the  iron  formation,  and  layers 
of  iron  formation  are  found  well  up  in  the  slate,  so  that  in  a  broad  way  the 
gradation  zone  may  be  several  hundred  feet. 

An  examii^ation  of  the  map  will  show  the  Virginia  slate  to  encroach 
on  the  south  margin  of  the  iron  formation  to  greatly  varying  distances, 
with  a  result  that  the  surface  outcrop  of  the  iron  formation  ranges  in  width 
from  2  or  more  miles  to  less  than  a  quarter  of  a  mile. 

This  might  be  explained  by  steeper  dips  at  the  narrow  places  than  at 
the  wide  places  in  the  iron  formation,  erosion  having  thus  vmcovered  less 
of  the  iron  formation  where  the  dips  were  steep.  This  is  probably  a  par- 
tial explanation  of  the  narrowness  of  the  iron  formation  belt  in  the  neigh- 
borhood of  Biwabik.  The  dips  are  on  an  average  somewhat  greater  at 
Biwabik  than  at  Mountain  Iron,  for  instance.  However,  the  difference  in 
dip  is  not  sufficient  to  account  for  the  extreme  narrowness  of  the  formation 
near  Biwabik,  nor  in  the  area  as  a  ^vhole  is  there  uniform  variation  of  the 
dip  sufficient  to  account  for  variation  in  width  of  the  formation. 

The  distribution  might  be  explained  by  the  greater  dip  of  the  plane 
of  surface  erosion,  either  atmospheric  or  glacial,  in  places  where  the  forma- 
tion is  wide  than  where  narrow,  the  greater  dip  of  the  surface  bringing'  it 
more  nearly  parallel  with  the  dip  of  the  iron  formation  and  thus  uncovering 
more  of  it.  While  this  may  help  to  explain  the  distribution,  the  dip  of  the 
the  plane  of  erosion  does  not  show  the  requisite  variation  in  inclination  to 
fully  account  for  the  observed  distribution. 

Finally  the  distribution  may  be  explained  by  the  lateral  gradation  of 
the  iron  formation  into  the  Virginia  slate.  This  is  believed  to  be  the 
principal  cause  of  the  present  distribution.  In  the  Biwabik  area,  where  the 
slates  encroach  on  the  soiithern  boundary  of  the  iron  formation,  the  lack  of 
sufficient  steepness  in  dip  of  strata  or  flatness  of  plane  of  erosion  leaves 
the  lateral  gradation  of  the  iron  formation  into  slate  as  the  only  alternative 
explanation  of  the  distribution  at  this  point.  Moreover,  in  following  the 
iron  formation  eastward  trom  the   Biwabik  mine  through  the  Cincinnati 


176  THE  MESABI  IRON-BEARING  DISTRICT. 

property  the  iron  formation  and  slate  are  actually  found  interbedded 
direct!}"  along  the  strike  of  the  iron  formation.  At  numerous  points  in 
the  district,  as  alreadv  noted,  the  iron  formation  becomes  interstratified  with 
slate  toward  its  upper  portion.  This  being  the  case  it  would  be  strange 
indeed  if  the  conditions  favorable  to  the  deposition  of  slate  had  been 
reached  at  exactly  the  same  time  all  over  the  range.  Indeed,  it  would  be 
more  reasonable  to  suppose  that  while  at  some  places  the  iron  fonnation 
alone  was  being  deposited,  at  other  places  iron  formation  and  slaty  layers 
were  being  alternately  deposited,  and  at  still  other  places  •  slate  alone  was 
being  deposited,  thus  giving  a  lateral  gradation  from  iron  formation  into 
slate. 

COMPARISON   OF  SLATE  OF  VIRGINIA  AND   BIWABIK  FORMATIONS. 

It  is  difficult  to  discriminate  the  Virginia  slate  from  slate  layers  in  the 
iron  formation  on  lithologic,  chemical,  or  textural  grounds,  for  each  of 
these  slates  has  a  variety  of  phases  and  some  of  them  are  common  to  both. 
But  while  in  an  individual  case  it  may  not  be  possible  satisfactorily  to  dis- 
criminate between  the  two,  in  general,  the  differences  are  believed  to  be 
somewhat  as  follows: 

(1)  The  Virginia  slate  has  a  predominance  of  grayish  tones,  while  the 
slates  interstratified  with  the  iron  formation  have  red,  brown,  or  black  tones. 

(2)  The  slate  in  the  iron  formation  is  probabl}'  more  frequently  broken 
into  small  parallelopiped  blocks,  or  is  more  likely  so  to  part,  than  the 
Virginia  slate.  Insufficient  observations  have  been  made  to  warrant  posi- 
tive statements,  but  in  this  opinion  the  writer  is  in  agreement  with  some  of 
the  leading  mining  engineers  on  the  range. 

(3)  The  slates  in  the  iron  formation  contain  abundant  griinerite,  while 
the  Virginia  slate  contains  almost  none  of  it. 

(4)  The  slate  in  the  iron  formation  contains  on  an  average  a  lower 
percentage  of  alumina,  a  higher  percentage  of  iron,  and  in  some  cases  a. 
higher  percentage  of  silica  than  the  Virginia  slate. 

In  the  mapping,  where  pits  or  drill  holes  have  shown  slate  of  consid- 
erable thickness  with  few  or  no  iron-formation  layers  in  it,  it  has  been 
mapped  as  Virginia  slate.  Where  the  slate  is  mixed  with  the  iron-forma- 
tion layers  in  considerable  quantity,  or  the  iron-formation  material  has 
been  found  to  the  south  of  it,  the  slate  has  been  included  in  the  iron  forma- 
tion.    (See  p.  14S.) 


THE  VIRGINIA  SLATE.  177 

STRUCTURE.  , 

0}Dportunities  for  studying  the  structure  of  the  Virginia  slate  in  situ  are 
so  few  that  if  the  observer  were  dependent  upon  such  observations  alone  he 
would  be  unable  to  make  any  statements  concerning  the  structure  of  the 
formation  beyond  the  fact  that  it  dips  at  low  angles  away  from  the  high 
land  adjacent.  However,  the  slate  is  a  conformable  part  of  the  Upper 
Huronian  series,  the  other  members  of  which  show  clear  evidence  of 
folding  and  fracturing.  The  Virginia  slate  must  have  partaken  in  this 
deformation.  The  statements  applied  to  the  structure  of  the  Upper 
Huronian  series  on  pp.  178-180  will  therefore  apply  to  the  Virginia  slate, 
but  with  one  modification.  Drill  holes  going  through  slate  into  iron 
formation  sometimes  reach  water  under  pressure,  indicating  that  the  water 
has  been  ponded  in  the  iron  formation  under  the  slate.  This  indicates  the 
relatively  pervious  character  of  the  iron  formation,  and  it  seems  likely 
that  in  the  gentle  folding  of  the  Upper  Huronian  series  the  brittle  iron 
formation  was  more  broken  than  the  soft  slate  and  thus  affords  freer 
passage  to  water  than  the  slate. 

THICKNESS. 

The  thickness  of  the  Virginia  slate  can  not  be  determined  in  the  Mesabi 
district.  The  drift  covering  is  thick,  mining  exploration  stops  to  the  south 
where  the  slates  are  encountered,  and  the  souther!  3^  extent  of  the  slate  belt 
is  thus  unknown.  To  the  south  of  the  range,  however,  there  is  a  low, 
swampy  area,  west  of  the  gabbro,  extending  southward  for  about  35  miles  at 
its  widest,  which  is  presumably  underlain  by  slate.  If  the  slate  occupies  all 
of  this  area,  it  must  have  a  vast  thickness.  In  the  neighborhood  of  Grunflint 
Lake  and  eastward  the  equivalents  of  the  Virginia  slate  and  their  upward 
extension  have  been  estimated  by  Grant"  to  have  a  thickness  of  2,600  feet. 
This  does  not  represent  the  total  thickness  of  the  formation,  but  simplj^  the 
part  not  covered  by  the  gabbro.  In  the  Penokee-Gogebic  district  the 
Upper  Huronian  slate  lying  above  the  iron  formation  has  a  present  maximum 
thickness  of  13,000  feet.**  Thus  the  Virginia  slate,  which  has  an  inconsid- 
erable thickness  in  the  district  covered  by  the  general  map  of  the  Mesabi 
range,  is  continuous  with  a  formation  extending  to  the  south  which  probably 
has  a  great  thickness. 

oGeol.  Nat.  Hist.  Survey  Minnesota,  Final  Rept.,  Vol.  IV,  p.  48fi. 
6Mon.  U.  S.  Geol.  Survey  Vol.  XIX,  p.  -!56. 

MON  XLIII — 03 12 


178  THE  MESABI  IRON-BEARING  DISTRICT. 

SECTION   IV.     STKUCTURE   OF   THE   UPPER   HUEOXIAlSr   SERIES. 

As  a  whole  the  Upper  Huronian  is  a  well-bedded  series  of  sediments. 
The  bedding  is  most  pronounced  in  the  middle  and  upper  horizons.  The 
beds  have  gentle  dips,  averaging  between  5°  and  20°,  though  locally 
greater  or  less,  in  southerly  and  southeasterly  directions  away  from  the 
older  rocks  forming  the  core  of  the  Giants  range,  but  locally  the  dips 
show  much  variation  both  in  degree  and  direction.  About  the  southerly 
projecting  tongue  of  the  Giants  range,  in  the  vicinity  of  Virginia,  Eveleth, 
and  McKinley,  the  dips  are  westerly  on  the  west  side  of  the  tongue, 
southerly  at  the  end  of  the  tongue,  and  southeasterly  on  the  southeast  side; 
that  is,  throughout  approximately  normal  to  its  periphery.  Even  more 
conspicuous  than  the  change  of  dip  at  such  a  place  are  the  minor  variations 
between  exposures.  Seldom  is  it  possible  to  get  two  identical  readings  in 
dip  at  exposures  of  rock  separated  by  even  short  intervals,  although  the 
direction  and  amount  of  the  dip  come  within  the  above  limits.  These  facts 
indicate  that  the  beds  of  the  Upper  Huronian  series  are  tilted  away  from 
the  core  of  the  Giants  range  in  directions  normal  to  its  trend,  and  that  the 
gentlv  tilted  beds  are  not  plane  surfaces,  but  are  gently  flexed.  By 
tabulating  and  comparing  the  dips  it  becomes  further  apparent  that  the 
greater  flexures  are  not  random  ones,  but  generally  have  their  axes  normal 
to  the  trend  of  the  range.  Examining  the  attitudes  of  the  beds  still  more 
in  detail,  it  appears  that  the  great  flexures  themselves  are  not  simple,  but 
have  many  subordinate  flexures,  some  of  them  transverse  to  the  major  ones. 
The  complexity  of  the  structure  may  be  likened  to  that  of  water  waves. 
On  the  great  swells  and  troughs  there  are  smaller  waves,  on  the  smaller 
waves  there  are  still  smaller  ones,  and  so  on  down  to  the  tiniest  disturbance 
of  the  surface.  While  perhaps  the  majority  of  the  minor  flexures  in  the 
Upper  Huronian  rocks  have  attitudes  similar  to  the  larger  ones,  many  of 
them  vary  greatly  in  direction.  They  may  be  observed  at  almost  any 
single  exposure  of  the  Upper  Huronian  series. 

"While  the  great  flexures  are  very  gentle,  involving  very  small  changes 
in  degree  and  direction  of  dip,  the  minor  flexures  superimposed  ujwn  the 
greater  ones  are  frequently  sharp  and  conspicuous.  The  local  dips  may 
varv  as  much  as  50°  within  a  few  hundred  feet  and  change  their  direction 
considerably.  Dips  as  high  as  45°  or  even  60°  may  be  seen  in  the  iron- 
formation  layers  in  some  of  the  open  pits  of  the  mines,  as  at  the  Stevenson, 


STRUCTURE  OF  THE  UPPER  HURONIAN.  179 

the  Sauntry-Alpena,  the  Kanawha,  and  the  Sparta.  A  series  of  dips  taken 
at  intervals  through  the  Mountain  Iron,  OHver,  and  Biwabik  open  cuts  are 
tabulated  in  connection  with  the  description  of  the  ore  deposits  (pp. 
225-226).  The  iron  formation  shows  more  minor  contortion  than  the  rest 
of  the  series,  because  of  the  great  chemical  changes  which  it  has  undergone, 
but  it  is  not  probable  that  there  is  any  great  difFerence  in  the  major  folding. 

Accompanying  the  tilting  and  minor  folding  ofi  the  Upper  Huronian 
series  there  has  been  a  very  considerable  amount  of  fracturing,  especially  in 
the  comparatively  brittle  Pokegama  and  Biwabik  formations.  Indeed,  it 
seems  likely  that  the  folds  of  the  two  lower  members  of  the  Upper  Huronian 
series  are  mainly  the  result  of  relatively  small  displacement  along  fractures, 
and  only  to  a  small  degree  the  result  of  the  actual  bending  of  the  strata  with- 
out breaking.  The  ponding  of  water  beneath  the  Virginia  slate  would  seem 
to  indicate  that  this  formation  has  been  less  fractured  than  the  iron  formation 
because  of  its  less  brittle  character,  and  had  thus  yielded  to  deformation  by 
actual  bending  rather  than  by  breaking.  On  almost  every  exposure  of 
Pokegama  and  Biwabik  formation  rocks  joints  and  minute  faults  are  to  be 
observed  cutting  almost  perpendicularly  across  the  bedding.  In  each  case 
the  joints  seem  to  make  up  two  or  more  systems  crossing  each  other  at  various 
angles,  but  such  sets  have  little  constancy  of  direction  in  widely  separated 
exposures,  unless  we  except  a  set  of  joints  which  at  a  number  of  places 
have  an  average  direction  of  somewhere  between  N.  60°  and  70°  E.— that 
is,  approximately  parallel  to  the  trend  of  the  range.  In  the  massive  rocks 
the  joints  are  clear  cut  and  continuous  for  considerable  distances.  In  the 
well-bedded  rocks,  as,  for  instance,  in  the  thin-bedded  portions  of  the  iron 
formation,  the  joints  are  usually  more  irregular,  less  continuous,  and  less 
conspicuous.  In  such  places  each  individual  bed  may  be  more  or  less 
jointed  without  reference  to  the  layers  above  or  below. 

The  displacement  or  faulting  along  joints  has  been,  in  general,  small. 
The  displacement  is  rarely  3  or  4  feet,  and  commonly  it  is  measured  by  a 
few  inches.  In  the  neighborhood  of  some  of  the  iron  deposits  of  the 
Biwabik  formation  certain  facts  doubtfully  indicate  a  greater  displacement, 
but  this  is  discussed  in  connection  with  the  ore  deposits. 

Certain  of  the  joints  and  faults  have  been  tilled  with  vein  quartz  and 
others  not.  It  is  rather  surprising  that  so  little  vein  quartz  is  to  be  observed. 
Where  present  in  the  harder  rocks,  where  the  joints  are  clear  cut  and 


180  THE  MESABl  IRON-BEARING  DISTRICT. 

continuous,  the  quartz  veins  appear  likewise.  In  the  well-bedded  portions 
of  the  iron-bearing  formation,  where  the  joints  are  irregular  and  discon- 
tinuous, the  distribution  of  the  vein  quartz  is  also  irregular  and 
discontinuous,  being  rather  in  a  confused  zone  than  a  well-defined  plane. 
After  the  Upper  Huronian  series  were  tilted  and  folded,  the  upper 
edges  of  the  series  were  eroded  away,  with  the  result  that  the  rock  surface 
is  now  irregular,  with  dips  corresponding  roughl^y  in  direction,  but  not  in 
degree,  with  those  of  the  underlying  rock  strata,  being,  in  general,  less 
steep. 

SECTIOX  V.     THICK?rESS   OF  THE   UPPER   HUEOISTEAX  SERIES. 

From  what  has  been  said  concerning  the  thickness  of  the  constituent 
members  of  the  Upper  Huronian  series,  it  is  apparent  that  accurate 
statements  of  the  thickness  of  the  Upper  Huronian  series  as  a  whole  are 
not  possible.  The  Pokegama  quartzite  varies  in  thickness  from  0  to  500 
feet  and  averages  perhaps  200  feet.  The  Biwabik  iron  formation  ranges 
in  thickness  from  200  to  2,000  feet  and  averages  perhaps  1,000  feet.  The 
Virginia  slate  extends  indefinitely  southward  beyond  the  limits  of  the  area 
under  investigation.  Within  the  area  studied  an  actual  thickness  of  nearlv 
400  feet  has  been  observed.  Assembling  these  figures,  it  appears  that  the 
maximum  figure  for  the  thickness  of  the  Upper  Huronian  series  within  the 
limits  of  the  district  mapped  may  be  3,000  feet,  while  the  average  may  be 
1,500  feet.  But  the  total  thickness  of  the  Virginia  formation,  including 
its  southward  extension,  is  probably  several  times  as  great  as  the  thick- 
ness of  the  two  lower  members  of  the  series  combined,  for  the  thickness 
of  the  formation  in  this  area  may  be  commensurate  with  its  thickness  in 
the  Penokee-Gogebic  area,  as  the  extent  of  the  flat  area  south  of  the 
range  would  seem  to  indicate. 

SECTION  XI.     RELATIONS   OF  THE   UPPER  HURONIAX   SERIES   TO 

OTHER   SERIES. 

The  Upper  Huronian  series  lies  iinconformabh'  upon  the  Archean  and 
Lower  Huronian  rocks.     The  proof  of  unconformity  is  as  follows: 

(1)  The  conglomerates  at  the  base  of  the  series  (listed  imder  the 
discussidii  of  the  Pokegama  quartzite)  contain  fragments  derived  from  the 
underlying  rocks. 


RELATIONS  OF  THE  UPPER  HURONIAN.  181 

(2)  There  is  discordance  in  dip.  The  underlying  formations,  where 
they  have  any  parallel  structure  at  all,  are  almost  vertical.  The  Upper 
Huronian  is  well  bedded,  with  a  low  dip.  Moreover,  in  approaching  the 
contact  no  change  of  dip  is  to  be  observed,  either  in  the  Upper  Huronian 
or  in  the  underlying  rocks. 

(3)  There  is  a  difference  in  the  amount  of  minor  folding,  fracturing, 
secondary  cleavage,  and  further  consequent  metamorphism  of  the  two  series, 
the  Upper  Huronian  being  much  less  afiPected  than  the  older  series. 

(4)  The  Upper  Huronian  belt  overlies  Archean  and  Lower  Huronian 
rocks  indiscriminately.  Near  Biwabik,  for  instance,  the  northern  edge  of 
the  Upper  Huronian  series  lies  diagonally  across  the  contact  of  the  Archean 
and  Lower  Huronian  rocks. 

(5)  The  Lower  Huronian  is  intruded  by  the  granite  making  up  much 
of  the  core  of  the  Giants  range.  The  Upper  Huronian  series  is  not  so 
intruded,  and,  moreover,  in  the  conglomerate  at  its  base  it  bears  fragments 
of  this  granite. 

The  Upper  Huronian  series,  in  ranges  12  and  13,  is  in  eruptive  contact 
with  the  Keweenawan  granite  and  gabbro,  as  fully  shown  in  the  section 
devoted  to  the  Keweenawan. 


CHAPTER  VI. 

KEWEENAWAN,  CRETACEOUS,  AND  PLEISTOCENE  ROCKS. 

SECTIOX  I.     THE  KEWEENAW  AX   ROCKS. 

DULUTH   GABBRO. 

A  portion  of  the  great  mass  of  Keweeuawau  gabbro  of  northern 
Minnesota  comes  within  the  limits  of  the  Mesabi  district.  The  northern 
edg'e  of  the  mass  hes  diagonally  across  the  eastern  end  of  the  district, 
extending  from  neai-  the  Duluth  and  Iron  Range  track,  in  range  14, 
northeasterly  through  ranges  13  and  12  to  Birch  Lake.  Tkrough  range 
14  the  gabbro  is  in  contact  with  Virginia  slate:  in  ranges  13  and  12  it  is 
in  contact  with  the  Biwabik  iron  formation,  and  north  of  Birch  Lake  it  is  in 
contact  with  Lower  Huronian  granite.  The  northern  edge  of  the  gabbro 
forms  a  conspicuous  northward-facing  escarpment  overlooking  the  low- 
lying  area  of  the  Virginia  slate  and  of  iron  formation  immediately  to  the 
north.  To  this  the  name  Mesabi  range  was  first  applied.  In  the  neigh- 
borhood of  Birch  Lake  the  gabbro  comes  well  up  on  the  crest  of  the  Giants 
range,  and  here  it  does  not  stand  above  the  adjacent  rocks. 

The  gabbro  exposures  show  a  parting  into  massi^'e  bands  usually  10 
to  20  feet  thick,  but  sometimes  ranging  down  to  a  few  inches.  As  certain 
layers  have  somewhat  different  textures  from  those  above  and  below,  it  is 
certain  that  the  stinicture  is  at  least  in  part  one  induced  in  the  gabbro  when 
it  first  cooled,  but  also  a  considerable  amount  of  the  parting  may  be  a 
secondary  phenomenon.  The  banding  of  the  gabbro  in  the  IMesabi  district 
may  be  well  observed  near  Allen  Junction,  where  it  dips  to  the  northeast 
at  an  angle  of  about  40°.  In  addition  to  the  parting  into  bands  the  gabbro 
is  cut  b)'  vertical  joints.  Occasionally  a  combination  of  the  joints  and 
banding  structures  causes  the  rock  to  weather  into  spheroidal  l)locks,  which 
at  first  glance  look  surprisingly  like  conglomeratic  bowlders.     This  feature 

182 


THE  KEWEENAW  AN  ROCKS.  183 

also  may  be  observed  in  the  gabbro  about  three-quarters  of  a  mile  north  of 
Allen  Junction. 

The  petrography  of  the  gabbro  of  northeastern  Minnesota  has  been 
exhaustively  described  by  Irving,"  Bayley/'  Winchell/  Grant/  Elftman/ 
Clements/  and  others,  and  the  part  which  comes  witliin  the  Mesabi  district 
shows  no  features  not  covered  in  these  descriptions.  In  characterizing  the 
gabbro  in  the  Mesabi  district  one  can  not  do  better  than  to  quote  a  brief 
summary  of  the  petrographic  character  of  the  gabbro  as  a  whole  by  Dr. 
U.  S.  Grant: 

Tlie  gabbro  is  a  coai-se-grained  aggregate  of  plagioclase,  which  is  near  labrado- 
rite;  augite,  which  is  often  diallagic;  olivine  and  magnetite,  with  occasionallv 
h\'persthene,  biotite,  hornblende,  and  minor  accessor}'  minerals.  In  general,  the 
mass  is  of  fairly  miiform  composition.  Variations,  however,  take  place  mainlv  in 
three  directions:  First,  by  increase  of  feldspar  the  rock  becomes  an  anorthosite; 
second,  by  increase  of  feldspar  and  olivine  a  forellenstein  is  formed;  third,  by  increase 
of  magnetite  masses  of  titaniferous  magnetic  iron  ore  originate.  Along  its  northern 
limit  the  gabbro,  while  at  times  assuming  a  liner  grain,  usually  preserves  its  coarse 
grain  and  granular  texture  to  its  contact  with  the  underlying  rocks,  s" 

The  gabbro  mass  has  been  generally  regarded  as  an  extrusion  forming 

the  base  of  the  Keweenawan,  but  recent  work  on  the  relations  of  the  gabbro 

to  adjacent  formations,  especially  near  Gunflint  and  Akeley  lakes,  has  led 

Van  Hise,  Clements,  Grant,  and  the  writer  to  the  behef  that  the  gabb-o  is 

a  laccolitic  intrusion. 

CONTACT  PHASES  OF  GABBRO. 

On  the  north  side  of  Birch  Lake  the  gabbro,  where  in  contact  with  the 
iron  formation,  shows  a  segregation  of  coarse  magnetite  octahedra  in  layers. 
Moreover,  at  this  point  biotite  and  the  orthorhombic  pyroxenes,  particularly 
enstatite  and  hypersthene,  become  more  abundant,  while  the  monoclinic 
pyroxenes  are  corresponding!}'  less  abundant.  These  features  are  common 
to  the  gabbro  contact  in  other  parts  of  northern  Minnesota. 

«Mon.  U.  S.  Geol.  Survey  Vol.  Y,  1S83. 

''Jour.  Geol.,  Vol.  I,  1893,  pp.  433-i56,  587-596,  688-716;  Vol.  II,  1894,  pp.  814-825;  Vol.  Ill, 
1895,  pp.  1-20. 

<=  Reports  of  Geol.  Nat.  Hist.  Survey  Minnesota,  latest  conclusion  in  Final  Kept. ,  Vols.  IV  and  V. 

''Engineers  Yearbook,  Univ.  of  Minn.,  1898,  pp.  49-62.  Bull.  Geol.  Soc.  Am.,  Vol.  XI,  1900, 
pp.  503-510;  also  in  Reports  of  Minn.  Surv.  (see  Chapter  II). 

«Am.  Geol.,  Vol.  XXI,  1898,  pp.  90-109,  175-188,  contains  full  bibliography;  Vol.  XXII,  1898, 
pp.  131-149. 

/Mon.  U.  S.  Geol.  Survey  Vol.  XLV. 

!7Contact  metamorphism  of  a  basic  igneous  rock,  by  U.  S.  Grant:  Bull.  Geol.  Soc.  Am.,  Vol.  XI, 
1900,  pp.  504-505. 


184 


THE  MESABI  IRON-BEARING  DISTRICT. 


North  of  Bircli  Lake  also  (see  fig.  7)  is  a  curious  contact  rock 
between  the  gabbro  and  Lower  Hm-onian  granite.  Between  the  coarse 
gabbro  and  the  normal  coarse  granite  is  a  narrow  zone,  perhaps  150  feet 
wide,  occupied  by  a  fine-grained  micaceous  rock,  varying  from  brown  to 
pink  in  color,  which  looks  in  places  like  a  fine-grained  gabbro  and  in  others 
like  a  fine-grained  g-ranite.  In  the  field  it  was  not  determined  whether  the 
rock  was  an  altered  gabbro,  an  altered  granite,  or  an  intermixture  of  the 
two,  and   microscopic   study  does  not  help  us   out  much.     The   rock   is 


R.12  W. 


LEGEND 


R.12  W. 


Scale 


0  K  K  mile 

Fig.  7.— Detail  map  showing  distribution  of  Lower  Huronian  granite,  Biwabik  formation,  Duluth  gabbro.  and  contact 
phase  of  granite  with  gabbro  on  the  north  shore  of  Birch  Lake. 

composed  mainly  of  feldspar,  largely  orthoclase,  although  so  obscured  by 
cloudv  alteration  as  to  make  accurate  determination  difficult,  biotite,  and 
quartz.  The  texture  is  granitic.  These  are  features  characteristic  of 
granite,  and,  whatever  the  rock  once  was,  it  should  probably  now  be  called 
a  granite.  However,  it  is  still  jiossible  that  the  rock  represents  a  fine- 
grained contact  phase  of  the  gabbro  for  the  reasons  that  (1)  the  character 
of  the  feldspar  is  doubtful,  (2)  biotite  is  one  of  the  characteristic  minerals 
developed  in  the  gabl)r(>  near  its  contact,  (3)  in  tlic  area  eastward  toward 


THE  KEWEENAW  AN  ROCKS.  185 

Grunflint  Lake  free  quartz  has  been  fonnd  in  the  gabbro  itself,  and  (4)  in 
this  eastern  area  also  the  exomorphic  effects  of  the  gabbro  contact  on  the 
granite  are  very  slight. 

DIABASE. 

There  are  in  the  Mesabi  district  certain  rocks  associated  with  gabbro 
which  are  not  covered  in  the  above  general  account.  In  range  13  expo- 
sures of  fine-grained  diabase  appear  in  the  SW.  J  sec.  25,  T.  60  N.,  R.  13 
W.,  and  in  the  central  and  northern  portions  of  sec.  35,  T.  60  N.,  R.  13  W. 
Bowlders  of  the  same  material  indicate  its  extension  for  several  miles  east 
and  west,  and,  taken  together  with  the  exposures,  indicate  a  belt  with  a 
possible  width  of  something  less  than  a  mile,  a  length  of  at  least  3  miles, 
and  jjrobabl)'  much  more,  and  a  trend  northeast  and  southwest — that  is, 
parallel  to  the  general  strike  of  the  formation  boundaries  in  this  part  of  the 
district.  The  diabase  is  a  fine-grained  dark-gray  rock  which,  under  the 
microscope,  shows  a  well-developed  ophitic  arrangement  of  plagioclase 
feldspar  crystals  and  the  presence  of  abimdant  hornblende  and  less  abun- 
dant ilmenite  and  magnetite.  The  diabase  corresponds  lithologically  to  the 
diabase  sills  intruded  in  the  iron  formation  in  the  neighborhood  of  Gunflint 
Lake,  and  there  supposed  to  be  either  offshoots  of  the  gabbro  or  intrusives 
b<ith  in  the  gabbro  and  adjacent  rocks.  The  trend  of  recent  opinion  is 
toward  the  former  conclusion.  In  the  SW.  J  sec.  25,  T.  60  N.,  R.  13  W., 
south  of  the  diabase,  drill  holes  have  recently  penetrated  altered  slate 
(cordierite-hornstone).  The  relations  of  the  slate  to  the  surrounding  rocks 
are  unknown  because  of  lack  of  exposures  and  exploration.  If  the  slate  is 
continuous  with  that  to  the  west,  which  had  not  heretofore  been  known  to 
extend  farther  east  than  sections  6  and  8  of  the  same  range,  the  diabase 
must  be  a  sill  intruded  in  the  Upper  Huronian  series.  If  the  slate  is  not 
continuous  with  the  main  belt  of  slate  to  the  west  it  must  be  an  isolated 
mass  in  the  Keweenawan  rocks,  and  the  diabase  would  belong  with  the 
main  mass  of  the  Keweenawan.  From  the  analogy  of  its  lithologic 
character  with  that  of  the  diabase  sills  to  the  east,  from  its  distribution,  and 
from  the  occurrence  of  slate  to  the  south,  it  is  thought  that  the  diabase  is 
probably  a  sill,  but  lack  of  exposures  and  sufficient  exploration  make  it 
quite  impossible  at  present  to  show  its  boundaries  on  the  map.  The  area 
to  the  south  of  the  diabase,  including  that  in  which  the  slate  has  been  found, 
is  therefore  mapped  as  Keweenawan. 


186  THE  MESABI  IRON-BEARING  DISTRICT. 

A  little  southeast  of  the  northwest  eoriier  of  sec.  34,  T.  59  X.,  R.  14  W., 
Mr.  E.  J.  Loiigyear  found  diabase  in  a  drill  hole  at  the  depth  of  984  feet, 
having  passed  through  IG  feet  of  drift,  392  feet  of  black  slate,  and  576  feet 
of  iron  formation.  Tliree  hundred  and  nine  feet  of  diabase  were  penetrated 
before  the  work  was  stopped.  The  iron  formation  is  bounded  on  the  north 
by  Lower  Huronian  gray  wackes  and  slates,  upon  the  eroded  edges  of  which 
lies  the  iron  formation,  with  perhaps  a  thin  layer  of  Pokegama  quartzite 
between.  The  fact  that  the  diabase,  rather  than  the  Pokegama  quartzite  or 
Lower  Huronian  graywacke  and  slate,  was  reached  bj^  the  drill  below  the 
iron  formation  would  be  in  accord  with  the  supposition  that  the  <liabase 
formed  a  sill  intruded  into  the  iron  formation  at  this  place. 

EMBARRASS   GRANITE. 

Through  ranges  12  and  13,  and  as  far  west  as  sec.  2  in  range  14,  a 
distance  of  15  miles,  the  granite  forming  the  core  of  the  Giants  range  is  intru- 
sive into  the  Upper  Huronian  series.  Whether  it  was  intruded  at  the  close 
of  the  Upper  Huronian  epoch  or  during  the  succeeding  Keweenawan  is  a 
matter  of  doubt,  and  indeed  is  a  matter  of  small  consequence.  The  fact 
that  granite  dikes  cut  the  Keweenawan  series  in  other  parts  of  northern 
Minnesota  makes  it  a  plausible  assumption  that  the  granite  was  intruded  in 
Keweenawan  time,  but  no  relations  of  the  granite  to  the  Keweenawan  have 
been  observed  in  the  Mesabi  district.  The  granite  is  named  the  Embarrass 
granite  from  its  lithologic  similarity  to  granite  exposed  at  Embarrass  station 
on  the  Duluth  and  Iron  Range  Railroad,  just  north  of  the  Mesabi  range. 

The  Embarrass  granite  is  a  pink  hornblende-granite.  It  is  usually 
of  coarse  grain,  but  shows  much  variation.  In  general  the  grain  l:)ecomes 
finer  toward  the  west.  The  characteristic  feature  of  the  granite  is  its  large 
content  of  quartz  in  small  and  large  grains,  which  are  verv  conspicuous, 
especially  on  the  weathered  surface.  The  quartzes  range  in  diameter  from 
over  a  centimeter  to  a  few  millimeters.  When  large  they  have  a  char- 
acteristic purplish-blue  color.  In  its  content  of  quartz  the  Eml)arrass 
granite  is  readily  distinguished  from  the  Lower  Huronian  g-ranite  in  the 
central  and  western  parts  of  the  range,  in  which  the  quartz  is  exceedingly 
rare  or  entirely  lacking.  Other  constitutents  arc  pink  orthoclase  feldspar, 
which  sometimes  occurs  as  prophyritic  crystals  almost  an  inch  long,  and  a 
rather  small  amount  of  liornblende.    The  relative  nbuudance  and  coarseness 


THE  KEWEENAW  AN  ROCKS.  187 

of  all  the  constituents  of  the  granite  of  course  show  the  usual  variations  of 
a  large  granitic  mass. 

Under  the  microscope  the  feldspar  is  observed  to  be  orthoclase,  rarely 
microcline.  Usually  it  is  fresh,  but  where  weathered  shows  cloudy  alter- 
ations to  kaolin  and  mica.  Zonal  growths  of  the  feldspar  grains  are 
couspiciious  features.  The  hornblende  is  of  the  common  green  variet}^. 
The  quartz  for  the  most  part  is  clear  and  limpid,  but  shows  undulatory 
.  extinction  and  minute  inclusions  in  irregular  aggreg'ates  and  in  lines.  In 
addition  to  the  essential  constituents  there  is  present  a  considerable 
quantity  of  sphene,  and  some  black  oxide,  which  from  its  common  associa- 
tion with  sphene  is  taken  to  be  ilmenite  rather  than  magnetite.  An 
occasional  crystal  of  andalusite,  and  perhaps  also  tourmaline  and  garnet, 
are  to  be  observed  (specimens  45076  and  45139). 

Cutting  the  gi-anite  are  a  few  dikes  of  finer-grained,  lighter-colored 
quartzose  granite,  which  under  the  microscope  is  found  to  differ  from  the 
one  just  described  only  in  lacking  hornblende  and  the  rare  elements 
mentioned. 

PEOOF     OF     INTRUSION    OF   EMBARRASS    GRANITE    INTO    THE    UPPER    HURONIAN    SERIES. 

Through  the  central  and  western  portions  of  the  Mesabi  district  thei'e 
is  abundant  evidence  that  the  Giants  Range  granite  is  older  than  the 
Upper  Huronian  rocks,  and  it  has  always  been  assumed  that  this  conclusion 
applied  to  the  eastern  portion  of  the  range  as  well.  The  first  conclusive 
proof  that  the  granite  of  the  eastern  portion  of  the  range  is  intrusive  into 
the  Upper  Huronian  series,  instead  of  older  than  it,  was  found  in  a  gorge 
in  the  NW.  i  of  NW.  i  sec.  17,  T.  60  N.,  R.  12  W.,  where  the  contact 
between  the  granite  and  the  iron  formation  is  well  exposed.  Figs.  6  and  7 
show  the  relations  of  the  two  formations.  The  proof  of  intrusive  relations 
at  this  point  may  be  briefly  summarized  as  follows: 

(1)  There  are  knife-edge  contacts  or  occasionally  a  gradation  zone  a 
fraction  of  an  inch  in  thickness,  as  shown  by  fig.  A  of  PI.  XVIII. 

(2)  At  these  contacts  there  is  no  trace  whatever  of  any  conglomeratic 
material  derived  from  the  granite. 

(3)  The  lower  part  of  the  iron  formation  is  penetrated  by  granite 
dikes,  and  irregular  layers  of  iron  formation  are  found  partly  sliced  off  by 
the  granite  (see  fig.  6).  At  one  place  a  mass  of  the  granite  cuts  across 
the  stratification  of  the  iron  formation  for  a  distance  of  5  feet  vertically,  and 


188  THE  MESABI  IRON-BEARING  DISTRICT. 

the  upward  continuatiou  of  the  granite  at  this  point  appears  as  a  dike  in 
the  upper  surface  of  the  iron  formation.  The  stratification  of  the  iron 
formation  has  suffered  a  little  disturbance,  for  it  dips  north  rather  than 
south  in  this  immediate  vicinity. 

(4)  The  iron  formation  in  contact  with  the  gi-anite  has  undergone 
considerable  metamorphism,  as  described  on  pages  161-163. 

(5)  At  the  contact  there  has  been  in  places  an  abundant  separation 
of   quartz  along  the  periphery  of  the  intrusive  gi-anite  or  between  the . 
layers  of  the  iron  formation. 

The  intrusion  of  the  granite  is  shown  by  the  relations  observed  in 
the  gorge  above  described  and  by  drill  records  to  have  been  mainly 
parallel  to  the  stratification  of  the  iron  formation,  which  lies  with  a  very 
gentle  dip  to  the  south,  but  in  a  minor  way  the  granite  has  broken  across 
the  stratification.  The  granite  is  coarsely  crystalline,  and  must  have  cooled 
far  below  the  surface,  when  the  iron  formation  was  deeply  buried. 

After  the  intrusive  relations  of  the  granite  and  Upper  Huronian  had  been 
worked  out  in  the  gorge  above  described  and  the  peculiar  quartzose  character 
of  the  granite  noted,  the  granite  of  the  Giants  range  both  east  and  west  was 
reexamined  to  see  how  much  of  it  is  of  this  character  and  how  much  older 
granite.  The  newer  granite  was  found  to  continue  eastward  as  far  as  Birch 
Lake  and  westward  to  sec.  2,  T.  59  N.,  R.  14  W.,  where  it  stops  abruptly. 
From  here  westward  the  granite  is  of  the  normal  Lower  Huronian  type. 

At  Birch  Lake  the  Embarrass  granite  and  Lower  Huronian  granite 
are  well  exposed  along  the  shores,  and  their  relations  may  be  satisfac- 
torily studied.  The  older  granite  occupies  much  of  the  north  shore  and 
extends  widely  northward  to  White  Iron  Lake  and  beyond.  It  is  identical 
in  character  with  the  Griants  Range  granite  through  the  central  and 
western  portion  of  the  Mesabi  district,  though  if  anything  it  is  a  little 
coarser.  Cutting  this  granite  are  younger  quartzose  granites  of  at  least 
two  periods  of  intrusion,  one  of  them,  the  finer  grained,  being  often  in  north 
and  south  trending  dikes  in  the  other.  The  coarser  quartzose  granite  is 
perhaps  the  counterpart  of  the  main  mass  of  Emban-ass  granite  above 
described,  and  the  finer  and  lighter  quartzose  granite  corresponds  to  the 
dikes  in  tlie  Embarrass  granite;  but  whether  or  not  this  correspondence 
holds,  the  quartzose  granite  in  general  is  clearly  a  part  of  the  Embar- 
rass granite  to  the  southwest,  and  is  intrusive  into  the  older  or  Lower 
Huronian  granite,  as  would  be  expected. 


KEWEENAWAN  AND  CRETACEOUS  ROCKS.  189 

SECTION  II.     CRETACEOUS  ROCKS. 

In  a  number  of  places  in  the  central  and  western  portions  of  the 
Mesabi  district  there  are  small,  thin,  isolated  patches  of  Upper  Cretaceous 
sediments  resting  uncomformably  upon  the  Upper  Huronian  formations. 
By  referring  to  the  map  such  areas  may  be  noted  near  ^^irginia,  Mountain 
Iron,  Buhl,  in  sees.  13  and  24,  T.  66  N.,  E.  24  W.,  and  in  sec.  31,  T.  57  N., 
R.  22  W.  Certain  deposits  of  blue  clay  in  various  parts  of  the  range,  well 
shown  on  the  high  hill  in  sees.  22  and  23,  T.  55  N.,  R.  26  W.,  may  also  be 
of  Cretaceous  age.  At  the  Oliver  mine  a  small  patch  of  conglomerate  may 
be  found  resting  on  the  ore.  The  Cretaceous  rocks  must  have  much  wider 
distribution  than  this,  but  thus  far  exploration  has  not  shown  them.  How- 
ever, while  they  probably  occupy  a  much  larger  area  than  is  indicated  on 
the  map,  they  do  not,  by  any  means,  cover  all  of  the  area,  as  is  shown  by 
the  numerous  explorations  passing  directly  from  drift  into  the  Upper 
Huronian  formations.  From  the  distribution  of  the  few  remnants  now 
known  it  is  certain  that  Cretaceous  rocks  once  overlaid  all  of  the  district 
west  of  range  16,  that  the}^  may  have  extended  further  to  the  east,  and  that 
erosion  has  largely  removed  them  from  the  area  they  did  occupy. 

The  rocks  consist  of  conglomerate  and  shale.  The  conglomerate,  in 
the  occurrences  known,  overlies  iron  formation  and  sometimes  iron  ore. 
As  would  be  expected,  therefore,  the  fragments  of  the  conglomerate  are 
derived  from  the  iron  formation.  They  consist  mainly  of  heavy  ferruginous 
chert  and  iron  ore,  both  hematite  and  limonite.  Except  locally,  and 
especially  when  the  pebbles  are  of  hard  material,  they  are  not  well 
rounded.  There  are  present  in  the  conglomerate  also  fossils  which  are 
described  below.  The  fragments  are  but  loosely  cemented.  When  broken 
out  of  the  ledge  the  rock  is  fairly  compact,  but  on  being  exposed  to  weath- 
ering it  soon  disintegrates.  The  cement  is  largely  ferruginous,  but  there 
is  present  also  a  considerable  amount  of  white  or  yellow  substance  which 
Mr.  A.  T.  Gordon,  chemist  of  the  Mountain  Iron  mine,  found  to  consist  of 
silica  and  alumina,  and  it  is  thus  essentially  a  cla}'.  Occasionally  there 
may  be  observed  also  minute  greenish-yellow  particles  in  the  cement 
which  may  be  glauconite  grains,  so  common  to  the  Cretaceous. 

The  shales  are  soft,  thin-bedded  rocks  of  a  bluish-gray  color  when  fresh, 
but  frequently  of  a  light  color  due  to  bleaching.     These,  too,  contain  fossils. 


190  THE  ME8ABI  IRON-BEARING  DISTRICT. 

FOSSILS. 

Selected  specimens  of  the  shale  aud  conglomerate  containing  fossils  were 
submitted  to  Mr.  T.  W.  Stanton,  paleontologist  of  the  U.  S.  Geological 
Survey,  for  examination.  He  pronounced  them  to  be  "unquestionably 
Upper  Cretaceous  forms — not  older  than  the  Fort  Benton,  and  probably 
not.  younger  than  the  Fort  PieiTe"  horizon.  Mr.  Stanton's  report  is  as 
follows: 

The  specimens  have  four  different  lot  numbers,  three  of  which  Mr.  Leith  informs 
me  refer  to  one  locality,  "Arcturus  mine,  in  sec.  24,  T.  56  N.,  R.  24  W.,  Mesabi  iron 
range,  Minnesota."'  These  are  all  fossiliferous.  The  specimens  numbered  -ioeiO  are 
similar  lithologically,  but  show  no  recognizable  fossils,  and  will  not  be  again 
referred  to. 

The  forms  recognized  in  each  lot  are  as  follows: 

No.  45573.  Mactra  sp.  A  small  form  closely  resembling  M.  gracilis  Meek 
and  Hay  den,  but  probably  distinct  species.  Inoceramm  sp.  An  immature  specimen 
that  may  be  young  of  I.  fragilh  Hall  and  Meek. 

No.  45576.  Mactra  sp.  Same  as  above.     Lunatia  sp.  A  single  small  cast. 

No.  45733.  Ostrea  sp.  One  or  possibly  two  small  species  of  oysters  not  specific- 
ally identifiable  with  any  described  form  from  the  western  interior.  Inoceramus 
sp.  A  fragmentary  small  specimen  like  the  one  in  No.  45573.  Mactra  sp.  Same  form 
as  in  the  other  lots.  Carclium  sp.  Fragmentary  imprint  of  the  surface  of  a  costate 
shell  probably  belonging  to  this  genus. 

These  fossils  are  unquestionably  marine  Upper  Cretaceous  forms,  but  the  exact 
horizon  represented  by  the  bed  yielding  them  has  not  been  determined  with  certainty. 
The  Inaceramus  may  be  the  young  of  /.  fmgiJls.  which  is  a  Fort  Benton  species, 
while  the  other  forms  belong  to  types  that  may  occur  in  any  of  the  Cretaceous 
horizons  above  the  Dakota.  On  the  basis  of  the  present  evidence  I  can  only  say  that 
the  horizon  is  not  older  than  the  Fort  Benton  and  probably  not  younger  than  the 
Fort  Pierre. 

In  1893  Mr.  H.  V.  Winchell,  of  the  Minnesota  survey,  submitted  fos- 
sils from  the  same  horizon  to  Dr.  C.  A.  White,  of  the  National  Museum. 
Dr.  White  referred  them  to  the  Colorado  formation  of  the  Upper  Cretace- 
ous.    His  report  is  as  follows : " 

I  have  examined  the  small  collection  of  fossils  which  you  obtained  from  the 
Mesabi  range  in  northern  Minnesota,  and,  although  they  are  all  in  an  imperfect  con- 
dition, I  do  not  hesitate  to  refer  them  to  the  Upper  Cretaceous.  The  following 
genera  are  represented  by  the  collection:  Ostrea,  hioct-mmm,  Modiola,  P/'niia, 
Yoldial,  Trigonurca,  Actaeon'i,  Troc/iush  and  Fasclolaria.  A  part  of  the  species 
represented  by  the  specimens  constituting  this  collection  are  evidently  new.  Some 
described  species  are  thus  represented,  and  others  are  probably  referable  to  described 
species,  but  they  are  too  imperfect  to  admit  of  satisfactory  determination. 

"Am.  Geol.,  Vol.  XII,  1893,  pp.  220-222. 


CRETACEOUS  AND  PLEISTOCENE  ROCKS.  191 

The  Yoldia  ?  referred  to  is  much  like  the  Y.  microdonata  of  Meek  and  Hayden. 
reported  to  have  come  from  the  Dakota  group  of  Kansas.  The  Tnoccramus  can  not 
be  distinguished  from  the  I.  fragilis  of  Hall  and  Meek.  This  is  a  characteristic  spe- 
cies of  the  Colorado  formation.  For  this  reason,  and  because  that  formation  is  known 
to  be  represented  at  other  localities  in  Minnesota.  T  think  there  is  little,  if  an}-, 
reason  to  doubt  that  the  deposit  from  which  you  obtained  these  fossils  represents  a 
portion  of  the  Colorado  formation  as  it  is  developed  in  the  great  interior  part  of  the 
continent. 

This  discovery  of  a  Cretaceous  deposit  at  a  point  so  far  to  the  northeast  in  Min- 
nesota is  a  matter  of  much  geological  interest,  and,  together  with  other  similar 
discoveries  in  that  State,  leaves  no  room  for  doubt  that  the  Cretaceous  sea  covered  a 
large  part,  if  not  all,  of  its  present  area. 

Later  Mr.  H.  V.  Winchell  submitted  other  fossils  from  northeni  Minne- 
sota, on  which  Dr.  White  reported  as  follows: 

In  addition  to  the  Tnoceramus^  Modiola,  and  one  or  two  others  that  were  sent  in 
the  first  lot  from  the  same  locality,  there  are  several  other  forms,  each  represented 
by  a  single  specimen,  as  follows: 

1.  Placenticeras  {Sjilienodiscus)  sp.  undet.  (Collected  by  Samuel  Sanford.) 
Part  of  the  inner  whorls  of  a  form  related  to  P.  (Sj^henodiscus)  leniiculare  Owen, 
from  which  it  differs  in  having  a  broader  umbilicus  and  more  simple  septa. 

2.  Pholadomya  *  *  *  Resembles  P.  suhventricosa  M.  and  H.,  though 
neither  the  type  nor  Mr.  Winchell's  specimen  is  well  enough  preserved  to  make 
the  comparison  satisfactory. 

3.  Barhatia    *     *     *     An  imperfect  cast. 
i.  A  reptilian  tooth. 

Ill  addition  to  the  fossils  above  noted,  the  Cretaceous  on  the  Mesabi 
has  been  found  to  contain  small  shreds  of  lignitic  material.  The  presence 
of  this  material  well  up  on  the  Mesabi  range  suggests  the  possibility  of 
finding  lignite  deposits  of  commercial  value  in  the  low  area  to  the  west, 
north,  or  south  of  the  Mesabi  range. 

SECTIOX   III.     PLEISTOCEKE   OB   GLACIAL   DEPOSITS. 

The  Mesabi  district  is  covered  by  a  mantle  of  glacial  drift,  of  the  late 
Wisconsin  epoch,  which  effectually  conceals  the  greater  part  of  the  Archean, 
Huronian,  Keweenawan,  and  Cretaceous  rocks  above  described.  On  lower 
slopes  the  drift  is  thick,  sometimes  reaching  1.50  to  200  feet,  and  here  of 
course  rock  exposures  are  rare;  on  middle  slopes  the  thickness  commonly 
does  not  exceed  50  or  60  feet,  and  20  to  50  would  measure  much  of  it;  on 
the  upper  slopes  of  the  range  the  drift  is  thin  or  altogether  lacking  and  rock 
exposures  correspondingly  abundant.     In  the  eastern  portion  of  the  district 


192  THE  MESABI  IRON-BEARING  DISTRICT. 

also,  where  the  Giants  Raiig-e  granite  has  a  higher  elevation  than  tn  the 
west,  the  drift  is  thin  and  allows  numerous  rock  masses  to  project  through; 
toward  the  west,  as  the  elevation  of  the  Giants  range  decreases,  the  drift 
becomes  thicker,  until  westward  from  Grand  Rapids  it  buries  even  the  crest 
of  the  Giants  range  to  a  depth  of  more  than  100  feet. 

Most  of  the  drift  in  the  Mesabi  district  may  be  classified  as  till.  It 
consists  of  bowlders  and  clay,  and  in  small  part  of  gravel  and  sand,  mingled 
indiscriminately.  The  bowlders  are  nearly  all  derived  from  the  Archean 
and  Hiu-onian  rocks,  either  immediately  subjacent  or  from  the  Giants  range 
to  the  north,  although  there  is  also  a  sparse  sprinkling  of  bowlders  deri\-e(l 
from  rocks  far  to  the  north  of  the  Mesabi  range.  The  most  numerous 
bowlders  are  granite  from  the  Giants  range,  but  southward  from  the  crest 
of  the  rano-e  these  become  mingled  with  the  various  rocks  of  the  Archean 
and  Huronian  formations.  Considering  the  number  and  character  of  the 
bowlders  from  the  rock  formations  of  the  Mesabi  district,  together  with  the 
lack  of  drift  along  the  crest,  it  is  apparent  that  higher  parts  of  the  district 
have  been  much  cut  down  by  the  invasion  of  the  ice. 

The  till  of  the  Mesabi  district  makes  iip  in  general  a  confused  morainal 
area,  marking  one  or  more  pauses  in  the  retreat  of  the  ice  toward  the 
north  or  northeast.  Potholes,  small  lakes,  and  swamps  are  characteristic 
features.  From  the  west  end  of  the  district  to  the  neighborhood  of  Hib- 
Ijing  the  till  forms  a  part  of  what  Upham"  has  called  the  Itasca,  or  Tenth, 
moraine  (the  tenth  one  back  from  the  most  southerly  moraine  left  by  the 
ice).  The  morainiil  till  from  Hibbing  to  the  Embarrass  River  represents 
the  merging  of  the  Itasca  moraine  and  the  Mesabi,  or  Eleventh,  moraine. 
Eastward  from  Embarrass  Ri^'er  to  Birch  Lake  the  till  belongs  to  the  Mesabi 
mi  iraine. 

While  parts  of  two  great  terminal  moraines  are  thus  represented  in  the 
Mesabi  district,  these  by  no  means  occupy  all  the  district  in  characteristic 
form.  There  are  considerable  areas  in  which  the  drift  shows  no  terminal 
morainal  t0]X)graph}-  and  perhaps  would  be  classified  as  ground  moraine, 
but  in  view  of  its  tliickness  and  association  in  short  distances  with  terminal 
morainal  features  tlie  general  statement  may  be  made  that  the  till  of  the 
Mesabi  district,  as  a  whole,  is  tenninal  morainal. 


"  Twenty-second  Annual  Report  Geol.  Nat.  Hist.  Survey  Minnesota,  1894,  pp.  49,  50.     See  also 
.\.  II.  KlftiniiM,  .\iii.  (U'nl.,  Vdl.  XXI,  1S08,  pp.  90-100. 


THE  GLACIAL  DEPOSITS.  193 

Tn  addition  to  the  till  or  unstratified  drift,  there  is  present  a  consider- 
able amount  of  stratified  drift — that  is,  drift  modified  by  water  resulting 
from  tlie  melting  of  the  ice — including  sand  and  coarse  kame  gravels. 

Probably  associated  in  origin  with  the  stratified  glacial  deposits  are  to 
be  mentioned  several  peculiar  gorges  at  high  elevations  which  cut  directly 
across  the  crest  of  the  Giants  range.  The  range  is  crossed  by  several 
marked  depressions;  the  Prairie,  Embarrass,  and  Dunka  rivers  occupy 
some  of  the  lower  and  major  ones.  But  in  addition  there  are  a  number  of 
small,  steep-walled  gorges  at  high  elevations  which  either  contain  no 
streams  or  contain  streams  very  small  as  compared  with  the  size  of  the 
valley.  They  vary  in  length,  depth,  and  width.  The  depressions  are  at 
present  of  U  shape,  although  tliis  may  be  due  to  glacial  or  stream  filling 
in  the  bottoms.  By  reference  to  the  general  map  of  the  district,  a  well- 
marked  gorge  may  be  observed  in  sees.  30  and  31,  T.  59  N.,  R.  18  W.,  with 
walls  standing  100  feet  above  the  level  of  the  present  bottom.  A  depres- 
sion across  the  range  may  be  observed  in  sec.  7,  T.  58  N.,  R.  16  W.  The 
Duluth  and  Iron  Range  Railway  follows  another  in  sees.  8  and  17,  T.  59  N., 
R.  14  W.  Another  may  be  observed  in  sections  9  and  16  of  the  same 
town  and  range.  Finally,  in  sec.  17,  T.  60  N.,  R.  12  W.,  is  a  small, 
well-marked  gorge  with  walls  standing  40  feet  above  the  present  bottom, 
and  exhibiting  well-marked  terraces  (see  fig.  5,  p.  160)  and  potholes.  The 
elevation  of  the  bottom  of  this  gorge  is  over  400  feet  above  the  level 
of  the  bottoms  of  some  of  the  deeper  depressions  crossing  the  range.  It 
is  apparent  that  depressions  of  such  widely  varying  elevations  must 
have  been  developed  under  different  conditions.  Some  of  the  lower 
and  deeper  ones,'  for  instance  that  occupied  by  the  Embarrass  Lakes,  are 
doubtless  mainlj'-  pre-Glacial  in  origin.  The  higher  ones,  it  is  believed, 
were  due  essentially  to  glacial  streams.  When  the  melting  of  the  great 
ice  sheet  caused  its  southern  margin  to  recede  to  the  north  of  the  Giants 
range  there  was  a  ponding  of  water  between  the  front  of  the  ice  sheet  and 
the  Giants  range,  and  when  the  level  of  the  water  was  high  enough  it  is 
likelv  that  the  water  discharged  through  the  lower  points  in  the  Giants 
range.  The  ridge  was  already  deeply  notched  by  pre-Glacial  streams,  but 
as  the  ice  withdrew  it  left  them  irregularly  filled  with  glacial  material,  so 
that  the  escaping  waters  were  compelled  to  make  new  channels  at  the 
lowest  points  of  escape.     Even  at  the  present  time  a  thickness  of  184  feet  of 

MON  XLIII — 03 13 


194  THE  MESABI  IRON-BEARING  DISTRICT. 

drift  has  beeu  found  in  the  Embarrass  Valley.  AVheu  thus  filled  the  steep- 
walled  gorg-es  at  high  elevations  in  the  Giants  range  were  carved.  At  the 
same  time  the  water  was  wearing  away  the  drift  bamers  in  the  old  stream 
channels.  As  soon  as  any  considerable  volume  of  water  was  diverted  into 
these  channels  tlie  gorges  in  the  rock  at  the  crest  of  the  range  were 
abandoned,  for  the  erosion  of  the  soft  di-ift  was  much  easier  than  that  of  the 
hard  rock  in  which  the  upper  gorges  were  being  carved.  The  disinterment 
of  the  old  drainage  channels  by  erosion  of  the  diift  has  continued  to  the 
present  time.  It  is  apparent  that  the  abandonment  of  the  i-ock  gorges  did 
not  occur  simultaneously  throughout  the  range,  for  some,  due  to  initial 
depression  or  to  the  structure  and  character  of  the  rock,  were  cut  deeper 
than  others  and  would  be  abandoned  later.  As  the  water  fell  in  the  great 
glacial  lake  north  of  ]\Iesabi  the  irregularities  of  the  bottom  may  have 
caused  the  water  to  be  divided  into  several  lakes,  among  them  Lakes 
Norwood  and  Dunka." 

On  many  of  the  rock  surfaces  of  the  Mesabi  distinct  glacial  striae  mark 
the  course  of  the  ice  across  the  area.  The  stride  show  minor  diversity  in 
direction,  but  in  general  they  range  from  S.  10°  W.  to  S.  30°  W.  At  Poke- 
gama  Falls  and  Prairie  River  Falls  there  is  greater  variation,  at  the  former 
place  the  direction  being  S.  50°  W.  and  at  the  latter  a  little  east  of  south. 

The  o-lacial  strise,  toarether  with  the  character  and  distribution  of  the 
fragments  in  the  drift  derived  from  the  cutting  down  of  the  Giants  range, 
show  that  the  movement  of  the  ice  was  essentially  from  northeast  to  south- 
west, as  noted  by  Upham.  Todd ''  has  maintained  that  the  ice  advanced 
into  northeastern  Minnesota  in  two  lobes,  one  through  Lake  Superior  and 
the  other  from  the  north  and  northeast  down  the  Red  River,  and  on  this 
hypothesis  the  morainal  material  on  the  j\Iesabi  may  in  part  represent 
interlobate  deposits.  While  the  northwestern  edge  of  a  Lake  Superior  lobe 
may  have  come  nearly  or  quite  up  to  the  Mesabi  range,  the  lno^•ement 
across  the  range  itself  was  unquestionably  from  the  northeast. 

"N.  H.  Winchell,  Bull.  Geol.  Soc.  Am.,  Vol.  XII,  1900,  p.  12.5. 
I'Am.  Geol.,  Vol.  XVIII,  1896,  pp.  225,  226. 


CHAPTER  VII. 

RESUME  OF  GEOLOGIC  DEVELOPMENT  AND 
CORRELATION. 

SECTION  I.— RESUME  OF  GEOLOGIC  DEVELOPME^STT. 

During  the  building  of  our  continent  there  have  been  many  general 
elevations  and  subsidences  of  vast  areas  with  reference  to  the  sea  level, 
resulting  in  the  alternate  pushing  back  and  encroachment  of  the  ocean. 
The  causes  of  these  movements  are  complex  and  need  not  be  discussed. 
That  they  have  actually  occui'red  is  shown  by  the  fact  that  the  continent 
is  largely  built  up  of  layers  of  marine  deposits,  between  some  of  which 
are  marked  discordances  of  structure  and  profound  differences  in  life 
remains.  The  Mesabi  district,  as  shown  by  the  facts  given  in  preceding 
chapters,  has  been  involved  in  some  of  these  general  movements  and  has 
been  covered  by  ocean  waters  at  several  times. 

The  oldest  rocks  of  the  Mesabi  district,  the  Archean  series,  are  of 
igneous  origin.  For  a  long  period  these  rocks  must  haA'e  had  the  district 
to  themselves,  for  there  is  evidence  that  the  rocks  had  been  mashed,  bent, 
and  broken  by  slow  earth  movements,  during  which  mountain  masses  were, 
perhaps,  formed  which  had  been  slowly  but  deeply  cut  by  surface  erosion 
before  the  next  overlying  series  was  deposited. 

The  erosion  of  the  area  was  accompanied  and  followed  by  subsidence 
and  by  encroachment  of  the  ocean,  which  deposited  the  Lower  Huronian 
sediments.  The  first  deposit  of  the  advancing  waters  was  conglomerate, 
made  up  of  fragments  from  the  Archean  rocks.  As  the  waters  grew  deeper 
over  the  Mesabi  district,  or  the  conditions  changed  in  other  ways,  finer 
detrital  debris  was  deposited,  which,  when  hardened  and  metamorphosed, 
became  graywacke  and  slate. 

After  a  time  sufficient  to  allow  the  deposition  of  fine-grained  Lower 
Huronian  sediments  to  a  thickness  of  5,000  feet  or  more,  there  was  an 

195 


196  THE  MESABI  IRON-BEAKING  DISTRICT. 

elevation  and  folding-  of  the  area  and  a  consequent  falling  back  of  the 
ocean  waters.  This  was  either  accompanied  or  followed  by  the  intrusion 
of  granite  and  the  metamorphism  of  the  Lower  Huronian  strata.  As  a 
result  of  the  elevation,  the  folding,  and  the  intrusion  of  granite,  niountani 
masses  wei'e  produced,  which  were  slowly  worn  down  by  erosion.  This 
sequence  of  events  is  evidenced  by  the  truncated  folds  in  the  Lower 
Huronian  strata.  A  vast  period  of  time  must  have  been  required  to 
accomplish  these  results. 

The  degradation  and  subsidence  of  the  area  and  the  encroacliment  of 
the  ocean  brought  on  conditions  for  the  deposition  of  the  Upper  Huronian 
series.  The  first  deposit  of  the  Upper  Huronian  sea,  as  usual,  was  one 
characteristic  of  shallow  waters,  conglomerate  and  sand  (Pokegaraa  torma- 
tion).  When  the  conditions  changed  iron  formation  and  mud  (Biwabik  and 
Virginia  formations)  were  deposited.  Between  the  deposition  of  the 
Pokegama  formation  and  the  deposition  of  the  Biwabik  and  Virginia 
formations  there  was  probably  another  slight  oscillation  of  the  land,  which 
perhaps  even  brought  the  Pokegams  formation  above  water,  for  we  find  at 
the  base  of  the  iron  formation  a  thin  layer  of  conglomerate,  indicating  a 
brief  period  of  shallow  waters  and  wave  action  just  preceding  the  deposi- 
tion of  the  iron  formation.-  The  Biwabik  iron  formation  was  in  the  main 
deposited  before  the  Virginia  formation,  but  the  change  from  one  to  the 
other  did  not  occur  evenly  tln-ough  the  range.  While  the  iron  formation 
was  still  being  deposited  in  the  western  part  of  the  area,  mud  was  being 
deposited  in  the  eastern  part,  and  furthermore,  unusual  currents  or  minor 
oscillations  were  carrying  mud  layers  into  the  area  in  which  the  main 
deposit  was  of  iron-formation  material.  This  sequence  of  events  is  shown 
by  the  gradation  of  the  iron  formation  into  slate  when  following  the  range 
east  from  the  central  part  of  the  district,  and  by  the  interstratificatit)n  of 
slate  layers  with  the  iron  formation  in  other  parts  of  the  district. 

The  Upper  Huronian  sea  may  not  have  covered  the  area  immediately 
north  of  the  Giants  range;  in  other  words,  the  Giants  range  may  have 
formed  the  northernmost  shore  line  uf  at  least  a  part  of  this  ocean,  for  notwith- 
standing repeated  search  on  the  north  side  of  the  Mesabi  range  and  in  the 
area  between  this  range  and  the  Vermilion  range,  and  even  in  the  Vermilion 
range  itself  westward  from  its  connection  with  the  Mesabi  range,  no  trace 
of  Upper  Huronian  sediments  has  been  found,  and  while  erosion  could  have 


RESUME  OF  GEOLOGIC  HISTOKY.  197 

accomplished  tliis  result,  the  fact  that  the  series  is  so  well  developed  to  the 
south  side  of  the  Giants  range,  and  its  continuation  is  entirely  lacking  north 
of  the  Giants  range,  suggests  the  possibility  of  a  shore  line.  The  fact  that 
the  area  north  of  the  Giants  range  is  underlain  by  Archean  and  Lower 
Huronian  rocks  would  seem  to  indicate  that  the  general  area  north  of  the 
Giants  range,  including  the  major  portion  of  the  Vermilion  disti'ict,  may 
have  been  an  anticline  and  above  water  during  Upper  Hiu'onian  deposition. 
On  the  other  hand,  the  Upper  Huronian  rocks  are  known  to  have  very 
widespread  distribution  in  the  Lake  Superior  country,  and  it  is  quite  possible 
that  they  may  have  entirely  covered  the  supposed  anticline  noi'thward 
from  the  Giants  range  and  have  since  been  removed  from  this  area  by 
erosion,  which  would  naturally  cut  down  the  anticlinal  area  first. 

If  there  was  a  shore  line  along  the  Giants  range  during  the  deposition 
of  the  Upper  Huronian  sediments,  the  sediments  would  have  had  a  slight 
initial  dip  to  the  south,  due  to  original  deposition  in  this  position  against 
the  shore.  It  is  probable,  however,  that  the  general  southerly  tilting  now 
shown  by  the  Upper  Huronian  strata  is  due  mainly  to  subsequent  folding. 

The  deposition  of  the  Upper  Huronian  series  was  followed  by  an 
elevation  and  slight  folding  and  a  pushing  back  of  the  ocean  waters,  and 
this  in  turn  by  a  long  period  of  induration  and  erosion.  For  most  of  the 
district  erosion  did  not  cut  through  the  Virginia  slate  before  the  advent  of 
the  Keweenawan,  but  at  the  east  end  of  the  district  the  slates  were  probably 
removed  and  the  iron  formation  exposed. 

In  Keweenawan  time  the  Duluth  gabbro  mass  of  northeastern  Minne- 
sota was  inti'uded  between  the  Upper  Huronian  series  and  any  sediments 
which  may  have  overlain  it  and  greatly  metamorphosed  the  rocks  with  which 
it  came  in  contact.  There  is  no  direct  evidence  of  Keweenawan  sediments 
and  surface  flows  having  overlain  the  Mesabi  district,  but  if  they  were  ever 
present,  as  they  may  have  been,  judging  from  the  facts  observed  in  other 
parts  of  the  Lake  Superior  region,  the  district  must  have  undergone  another 
submergence  in  Keweenawan  time  prior  to  the  intrusion  of  the  gabbro. 

The  gabbro  is  now  found  to  lap  diagonally  over  the  Virginia  slate  and 
the  iron  formation  and  to  rest  on  the  eroded  edges  of  both,  showing  that 
the  post-Huronian  erosion  had  cut  through  the  slate  for  a  part  of  the 
district,  as  above  noted.  If  such  erosion  had  not  occurred,  it  would  be 
necessary  to  suppose  the  gabbro   to  have   been  intruded  along  an  even 


198  THE  MESABI  IRON-BEARING  DISTRICT. 

plane  geutly  inclined  to  the  stratification  of  tlie  Upper  Hurouian  series. 
Moreover,  the  relation  of  this  erosion  plane  to  the  Upper  Huronian  strata 
is  such  as  to  indicate  that  the  tilt  of  the  strata  was  either  to  the  south  or  to 
the  west  prior  to  the  erosion  which  pared  down  the  surface. 

The  g'abbro  intrusion  was  followed  by  an  intrusion  of  granite  in  the 
eastern  end  of  the  district,  which  greatly  metamoi-^jhosed  the  Upper  Huronian 
strata  with  which  it  came  into  contact.  The  time  of  the  intrusion  is  post- 
Huronian,  and  in  ^-iew  of  the  fact  that  granites  have  been  found  cutting  the 
Keweenawan  (and  not  the  Cambrian)  in  other  areas,  it  is  thought  that  the 
intrusion  in  the  Mesabi  district  occurred  in  Keweenawan  time. 

At  some  time  after  the  Keweenawan,  and  before  the  deposition  of  the 
Cambrian,  the  Upper  Huronian  and  Keweenawan  rocks  (and  of  course  all 
underlying  rocks)  were  folded.  This  folding  was  mainty  responsible  for  the 
gentle  southward  tilting  of  the  Upper  Huronian  strata  of  the  Mesabi  district. 
It  was  also  responsible  for  the  final  elevation  of  the  area,  the  erosion  remnant 
of  which  is  now  known  as  the  Giants  range.  Evidence  that  the  main 
folding  of  the  Upper  Huronian  occurred  in  post-Keweenawan,  rather  than 
pre-Keweenawan  time,  is  lacking  in  the  Mesabi  district  itself;  in  fact,  the 
only  positive  evidence  there  to  be  observed  is  that  the  Upper  Huronian  series 
was  slightly  folded  and  truncated  before  Keeweenawan  time,  as  shown  by 
the  relation  of  the  formation  to  the  Keweenawan.  But  in  other  parts  of  the 
Lake  Superior  country  the  two  series  have  been  uniformly  folded  together 
and  owe  their  present  attitudes  essentially  to  this  folding,  and  it  is  not 
probable  that  the  limited  area  of  the  Mesabi  alone  escaped  this  movement. 

During  Paleozoic  and  most  of  Mesozoic  time,  represented  in  other 
districts  by  deep  accumulations  of  sediment,  the  Mesabi  district  was 
probably  above  water  and  undergoing  erosion,  for  we  find  no  traces  of  such 
deposits  in  this  or  adjacent  districts.  Yet  it  is  not  certain  that  the  Cam- 
brian may  not  have  covered  the  area,  for  it  is  known  to  have  a  widespread 
distrilnition  in  the  Lake  Superior  country  and  to  have  been  stripped  of 
great  areas  by  erosion.  It  is  now  found  in  areas  almost  as  high  as  the 
Giants  range,  as,  for  instance,  the  jMenominee  district  of  Michigan.  Toward 
the  close  of  Mesozoic  time,  in  the  Cretaceous  period,  there  came  a  sub- 
sidence of  the  area  with  reference  to  the  ocean  level,  and  the  Cretaceous 
ocean  encroached  over  the  part  of  the  area  wt'stward  from  A  irgiuia  and 
])robably  extended  eastward  from  Virgiiii;!,  but  how  t';ir  no  evidence  is  as 
yet  at  hand  to  sliow. 


RESUME  OF  GEOLOGIC  HISTORY.  199 

With  the  subsequent  emergence  of  the  Mesabi  district  from  the  Creta- 
ceous ocean,  the  district  appeared  in  approximately  its  present  condition  so 
far  as  rock  succession  and  structure  are  concerned. 

After  another  long  period,  represented  in  other  parts  of  the  United 
States  b}'  deposition  of  Tertiary  sediments,  during  which  erosion  greatly 
modified  the  surface  of  the  land,  the  great  North  American  continental  ice 
sheet  of  Pleistocene  age  pushed  its  southern  margin  orer  the  Mesabi 
district.  The  granite  of  the  Griauts  range  bore  the  brunt  of  the  attack  from 
the  north  and  consequently  was  much  cut  down  by  the  ice,  but  all  the 
other  formations  of  the  district,  including  the  iron  formation,  were  truncated 
to  a  considerable  depth.  This  is  shown  by  the  abundant  drift  in  the  district 
largely  composed  of  local  material.  The  drift  for  the  most  part  conceals 
the  older  rocks  along  the  southern  slopes.  When  the  melting  of  the  ice 
sheet  had  caused  the  margin  to  recede  north  of  the  Giants  range,  it  is 
probable  that  a  considerable  body  of  water  was  ponded  between  the  ice 
front  and  the  crest  of  the  Giants  range,  and  when  high  enough  the  water 
escaped  through  the  lower  places  in  the  range,  and  in  so  doing  gouged  out 
some  of  the  remarkable  small  steep-walled  gorges  now  to  be  observed  at 
a  number  of  points  in  the  granite  at  the  crest  of  the  range. 

Since  Glacial  time  ordinary  erosion  agencies  have  been  at  work. 
Where,  as  in  the  upper  and  eastern  portions  of  the  range,  glacial  drift  is  thin 
or  lacking,  drainage  channels  have  followed  the  old  ones  formed  before  the 
Glacial  epoch.  On  the  lower  slopes  of  the  range,  and  particularlj-  in  the 
western  portion,  the  drainage  has  been  feebly  struggling  to  perfect  itself 
in  the  great  irregular  mass  of  glacial  ddbris,  but  thus  far  with  little  success. 

Since  the  emergence  of  the  Upper  Huronian  series  from  the  sea  and 
the  removal  of  the  Virginia  slate,  the  rocks  of  the  Biwabik  iron-bearing 
formation  have  been  continuously  undergoing  alterations,  part  of  which 
have  resulted  in  the  concentration  of  the  iron-ore  deposits  of  tlie  district. 
The  nature  and  progress  of  this  alteration  are  discussed  in  Chapter  VIII. 

Such,  in  brief,  is  the  history  of  the  district  as  shown  by  the  succession, 
structure,  and  relations  of  the  rock  strata  It  is  apparent  that  the  develop- 
ment of  the  Giants  range  has  been  a  complex  process.  Its  first  recognizable 
elevation  occurred  at  the  time  of  the  Lower  Huronian  folding  and  intrusion 
of  P'ranite.  At  the  time  of  the  inter- Huronian  erosion  the  elevation  was 
greatly  reduced,  and  the  Upper  Huronian  sea  may  have  entirely  submerged 
it.     Yet  it  is  possible  that  it  may  have  remained  sufficiently  high  to  form 


200  THE  MESABI  IRON-BEARING  DISTRICT. 

the  shore  Hue  of  a  portion,  at  least,  of  the  Upper  Huronian  sea.  Some  of 
the  spurs  of  Archean  and  Lower  Huronian  rocks  also,  as,  for  instance,  the 
one  ruunine"  out  toward  Eveleth,  niav  have  existed  either  above  or  below 
water  during  the  time  the  Upper  Huronian  sea  was  depositing  sediments  in 
the  area.  After  the  deposition  of  the  Upper  Huronian  the  range  was  again 
elevated  and  gentl}^  folded.  Before  the  Keweenawan  it  was  considerably 
reduced  by  erosion,  for  in  the  east  end  of  the  district  the  Keweenawan 
gabbro  rests  upon  the  truncated  edges  of  the  Upper  Huronian  rocks. 
After  Keweenawan  time  the  range. was  further  folded  and  uplifted.  Indeed, 
its  major  elevation  was  probably  developed  at  this  time.  From  Kewee- 
nawan to  Pleistocene  time  erosion  cut  down  the  range,  except  during-  the 
Cambrian  and  Cretaceous  periods,  when  the  process  was  perhaps  replaced 
by  deposition.  In  the  Grlacial  epoch  the  range  was  further  cut  down  by 
glacial  erosion.  And,  finally,  since  Pleistocene  time,  subaerial  erosion  has 
perhaps  very  slightly  reduced  the  portions  of  the  range  not  covered  by 
glacial  drift. 

SECTION  II.     CORRELATION. 

In  the  Lake  Superior  region  R.  D.  Irving,  followed  by  C.  R.  Van  Hise, 
and  their  assistants  on  the  United  States  Geological  Survey,  have  discrim- 
inated four  great  pre-Cambrian  series: 

(1)  The  Basement  Complex  or  Archean,  consisting  mainly  of  igneous 
rocks,  but  containing  also  sedimentary  rocks  in  small  quantity. 

(2)  The  Lower  Huronian,  a  sedimentary  series  resting  unconformably 
upon  the  Archean. 

(3)  The  Upper  Huronian,  another  sedimentary  series  resting  with  well- 
marked  unconformity  upon  both  the  Archean  and  Lower  Huronian. 

(4)  The  Keweenawan,  a  series  of  intercalated  lavas  and  sediments 
resting  unconformably  upon  all  the  underlying  rocks. 

The  Huronian  and  Keweenawan  series  together  make  up  the  Algonkiau 
system.  For  a  full  discussion  of  these  great  series,  the  development  of 
knowledge  concerning  them,  their  relations,  their  subdivisions,  and  their 
local  names,  the  reader  is  referred  to  the  reports  of  the  United  States 
Geological  Survey." 

The  series  above  named  had  been  studied  in  detail  and  their  relations 

«See  particularly  Bull.  86,  and  Sixteenth  Ann.  Rept.,  Pt.  I,  pp.  780-807;  see  also  correlation 
chapters  in  Monographs  XIX,  XXVIII,  XXXVI,  and  XLV. 


COERELATION.  201 

determined  in  the  other  iron-bearing  districts  of  the  Lake  Superior  region 
before  the  Mesabi  district  had  been  s^^stematically  studied  by  the  Survey. 

In  the  Mesabi  district  the  gabbro  has  been  assigned  by  all  to  the 
Keweenawan.  The  upper,  flat-lying  series  of  the  Mesabi  district  has  been 
generally  recognized  as  the  equivalent  of  the  Animikie  series  of  the  inter- 
national boundary,  eastward  fi-om  Akeley  and  Gunflint  lakes  to  Thunder 
Bay,  and  has  indeed  been  called  Animikie  by  nearly  all  who  have  had 
occasion  to  refer  to  the  series.  (See  Chapter  II.)  Irving  and  Van  Hise 
have  correlated  the  Animikie  series  with  the  original  Upper  Huronian 
series  as  worked  out  on  the  north  shore  of  Lake  Huron.  This  correlation 
has  been  consistently  followed  by  the  United  States  Geological  Survey  in 
its  work  on  the  Lake  Superior  region,  although  the  correctness  of  the 
correlation  is  disputed  by  a  number  of  the  Canadian  and  Minnesota  geol- 
ogists, who  maintain  that  the  Animikie  series  is  later  than  the  true  Upper 
Huronian.  The  controversy  as  to  the  equivalence  of  the  Animikie  and  the 
Upper  Huronian  will  not  be  gone  into,  but  the  correlation  by  the  United 
States  Geological  Survey  will  be  accepted  as  a  basis  for  the  correlation 
and  naming  of  the  Animikie  and  underlying-  series  in  the  Mesabi  district. 
Professor  Van  Hise  has  now  in  preparation  a  final  monograph  on  Lake 
Superior  geology  in  which  the  question  of  the  correlation  of  the  Animikie 
Avill  be  fully  discussed  in  the  light  of  recent  field  work  in  the  Lake 
Superior  region. 

The  rocks  underlying  the  "Animikie"  or  Upper  Huronian  of  the 
Mesabi  district  were  lumped  together  as  Keewatin  and  described  as 
essentially  igneous  by  the  geologists  of  the  Minnesota  survey,  and  largely 
because  of  this  were  formerly  correlated  with  the  Basement  Complex 
or  Archean  by  the  United  States  Geological  Survey.  When  systematic 
work  was  done  in  the  Mesabi  district  by  the  United  States  Geological 
Survey  it  was  found  that  the  supposed  Archean  Complex  or  Keewatin 
series,  underlying  the  Upper  Huronian,  really  consists  of  two  series,  one 
igneous  and  the  other  sedimentary,  which  correspond  respectively  in  their 
relations  and  lithological  character  with  the  Archean  and  Lower  Huronian 
series  of  other  parts  of  the  Lake  Superior  region,  particularly  the  Vermilion 
district  of  Minnesota. 

Thus,  each  of  the  four  great  divisions  of  the  pre-Cambrian  previously 
worked  oiit  by  the  United  States  Geological  Survey  for  the  Lake  Superior 


202  THE  MESABI  IRON-BEARING  DISTRICT. 

region  as  .a  whole  was  found  to  be  represented  in  the  Mesabi  district. 
Perhaps  in  no  otlier  district  is  the  proof  of  their  existence  and  relations  any 
more  decisive  tlian  here,  and  the  Mesalji  may  well  serve  in  these  features 
as  the  tj-pe  pre-Cambrian  district  of  the  Lake  Supei-ior  region. 

If  the  Minnesota  survey  had  divided  the  Keewatin  of  the  Mesabi 
district  into  the  Upper  and  Lower  Keewatin,  as  suggested  by  Grant,  and 
had  included  the  separately  mapped  greenstones  and  mica-schists  in  the 
Lower  Keewatin,  it  may  have  been  that  the  succession  of  the  Minnesota 
survey  would  have  been  identical  geologically  with  that  of  the  United 
States  Geological  Survey.  It  may  be  asked,  then,  why  the  terras  "Upper 
Keewatin,"  "Lower  Keewatin,"  and  "Tacouic"  or  "Animikie"  should  not 
have  been  retained  for  the  j\Iesabi  formations.  A  perusal  of  the  summaries 
of  literature  in  Chapter  II  will  show  that  these  terms  have  been  used 
from  time  to  time  and  from  place  to  place  in  a  great  ^variety  of  senses. 
As  at  jjresent  used  in  northern  Minnesota,  Upper  Keewatin  and  Lower 
Keewatin  each  include  rocks  which  the  United  States  Geological  Survey 
has  designated  as  Archean  and  Lower  Huronian.  Moreover,  the  terms 
"Hnronian"  and  "Archean"  had  been  applied  to  similar  series  in  other 
parts  of  the  Lake  Superior  country  before  the  term  "Keewatin"  was  iuti'o- 
duced,  and  should  therefore  have  been  retained  for  Minnesota.  The  term 
"Taconic"  has  been  used  as  an  equivalent  to  Lower  Cambrian,  and  includes 
both  Keweenawan  and  Animikie  rocks.  The  work  of  the  United  States 
Geological  Survey  has  demonstrated  the  presence  of  a  great  unconformity 
between  the  Cambrian  and  the  Keweenawan,  and  between  the  Keweenawan 
and  the  Animikie,  and  hence  the  term  "Taconic"  could  not  be  retained 
for  these  two  unconformable  series,  even  if  the  term  "Animikie"  had 
not  priority  in  the  Lake  Superior  region  and  the  term  "Tacouic"  wholly 
discredited  in  the  eastern  United  States,  where  the  term  was  first  applied. 

The  remarkable  similarity  of  the  Upper  Huronian  of  the  j\Iesabi  dis- 
trict to  that  of  the  Penokee-Gog'ebic  district  has  been  often  remarked. 
The  succession  within  the  series  is  the  same.  The  relations  to  adjacent 
series  are  similar.  Scarcely  a  phase  of  rock  in  one  series  can  not  be 
matched  in  tlie  other,  although  in  varying  abundance.  The  Upper  Huro- 
nian of  the  Mesabi  district  is  on  the  north  shore  of  Lake  Superior  and  dips 
to  the  south  at  a  low  angle.  The  Upper  Huronian  of  the  Penokee-Gogebic 
district  is  on  the  south   side   of  Lake   Su))erior  and  dips  to  the  north  at  a 


CORRELATION. 


203 


high  angle.  These  facts,  taken  together  with  the  s^mchnal  structnre  of 
the  Lake  Superior  Basin,  indicate  that  the  Upper  Hnronian  series  of  the 
Mesabi  and  the  Penokee-Gogebic  districts  form,  respectively,  the  nortli  and 
south  limbs  of  a  great  basin  into  which  the  Upper  Huronian  series  has 
been  folded  and  within  the  limits  of  which  also  the  series  was  probably 
originally  dejDOsited. 

The  hypothetical  course  of  the  truncated  edges  of  the  Upper  Huronian 
strata  connecting  the  two  districts  is  indicated  by  the  broken  line  in  fig.  8. 
The  area  south  of  the  Mesabi  range  is  low  and  swampy  for  a  considerable 


Cambrian. 


W^ 


mmm 


Kewe 


enawati 


Upper  Huron- 
ia2i(iron-\)earirLtf 
series)inclucling 
small  areas  of 
ArcKeajL  and 
LoTver  Hau'onian 
roclis 


Lo"wer 
Huronian 


Grajiite 


ArclieaTi. 
greeristoTLe 

Fig.  S. — Sketch  showing  possible  connection  of  the  Mepahi  with  Penokee-Gogebic  iron-bearing  series. 

distance,  and  this  area  is  presumably  underlain  by  the  Upper  Huronian 
slate,  which  is  soft,  easily  eroded,  and  has  a  synclinal  structure.  In  the 
vicinity  of  Carlton  and  Cloquet,  on  the  St.  Louig  River,  are  graywackes  and 
slates  standing  on  edge,  which  resemble  the  Lower  Huronian  series  of  the 
Mesabi  district  and  are  presumably  themselves  of  Lower  Huronian  age. 
About  12  miles  east  of  Aitken,  Minn.,  south  of  the  town  of  Kimberly,  are  two 
exposures  of  quartzite  identical  in  character  with  the  Pokegama  quartzite 
of  the  Mesabi  range.  To  the  south  of  this  quartzite  is  greenstone  identical 
with  the  Archean  greenstone  of  the  Mesabi  district.  To  the  west  and 
south  outcrops  of  granite  and  greenstone,  presumably  of  Lower  Huronian 


204  THE  MESABI  IRON-BEARING  DISTRICT. 

aud  Archean  age,  are  known  at  a  number  of  localities,  as  shown  on  the 
map.  The  Upper  Hnronian  series  for  this  part  of  the  region  may  lie 
within  the  limits  of  the  area  thus  outlined  by  the  Lower  Huronian  and 
Archean  rocks,  and  the  outcrop  of  quartzite,  which  is  presumably  Upper 
Huronian,  would  indicate  that  the  southern  arm  of  the  Upper  Huronian 
syncline  actually  comes  up  close  to  the  edge  of  the  ai-ea  so  outlined.  To 
the  north  of  the  quartzite  exposure  at  Kimberly  referred  to  is  iron-formation 
debris  in  the  di'ift  and  a  faint  line  of  magnetic  attraction  extending  in  a 
direction  nearly  east  and  west  from  the  quartzite.  Moreover  a  faint  line  of 
magnetic  attraction  has  been  carried  some  distance  west  of  Grand  Rapids. 
It  must  be  assumed  that  the  Upper  Huronian  belt  passes  north  of  Carlton 
and  Cloquet.  Its  course  in  the  area  between  these  places  and  the  Penokee- 
Gogebic  district  is  not  shown  by  any  direct  evidence,  but  it  seems  prob- 
able that  it  must  connect  with  the  Penokee-Gogebic  district  by  a  com- 
plex fold  for  this  reason:  The  occurrence  of  the  Keweenawan  and  Upper 
Huronian  rocks  in  the  Penokee-Gogebic  district  and  other  parts  of  the 
Lake  Superior  region  indicates  that  the  two  series  received  their  major 
folding  together,  and  the  folds  or  tongues  in  the  Keweenawan  rocks  shown 
on  the  map  are  likely  to  have  their  counterparts  in  the  Upper  Huronian 
strata  below.  The  truncated  edges  of  the  Upper  Huronian  strata  may 
therefore  appear  in  somewhat  the  position  shown  on  the  map.  But  where 
the  distribution  of  the  series  can  be  worked  out  the  series  is  known  to  pass 
under  the  Keweenawan  for  considerable  distances,  as  in  parts  of  the  Mesabi 
and  Penokee-Gogebic  districts,  and  it  is  more  than  likely  that  through  the 
unknown  connecting  area  the  iron  formation  is  actually  covered  by  the 
Keweenawan  over  considerable  areas.  Also,  during  the  erosion  period 
between  Upper  Hui'onian  and  Keweenawan  time,  the  entire  Upper  Huro- 
nian series,  including  the  iron-bearing  formation,  may  have  been  wholly 
removed  for  long  distances,  and  thus  the  Keweenawan  lies  against  the 
Lower  Huronian  or  the  Archean  without  intervening  series  equivalent  to 
the  Mesabi.  But  so  far  as  the  Upper  Huronian  still  occurs  at  the  sur- 
face it  is  likely  to  be  found  close  to  the  edge  of  the  Keweenawan,  and  for 
this  reason  the  dotted  line  is  drawn  nearly  parallel  to  the  periphery  of  the 
Keweenawan. 

That  the  Upper  Huronian  strata  of  the  Mesabi  and  Penokee-Gogebic 
districts  were  originalK'  connected  no  one  will  doubt  who  examines  tlieni 


CORRELATION.  205 


and  realizes  their  identity  in  character.  That  the  eroded  edges  of  the 
Upper  Huronian  folds  have  the  position  indicated  on  the  map  is  largely 
hypothetical,  but  is  in  accord  with  the  known  facts  and  inferences  derived 
from  them.  Exploitations  in  search  for  the  iron-bearing  member  of  the 
Upper  Huronian  series  beyond  the  known  distribution  in  the  Mesabi  and 
Penokee-Gogebic  disti'icts  ought  first  to  be  made  on  some  such  assumption 
as  to  distribution  and  carried  into  other  areas  as  the  developed  facts 
warrant. 


CHAPTER    VIII. 

THE  IRON-ORE  DEPOSITS. 

DISTRIBUTION". 

On  the  accompanying  geologic  map  of  the  Mesabi  district  the  principal 
properties  underlain  by  iron  ore  of  present  commercial  value  are  indicated 
by  shading  and  names.  The  ore  deposits  do  not  occupy  all  the  area  shaded, 
but  each  of  the  shaded  areas  contains  some  iron  ore.  The  following 
features  of  distribution  of  the  iron  ore  may  be  especiall}^  noted:  The 
deposits  are  numerous  through  the  central  portion  of  the  district,  and  they 
are  altogether  lacking  in  the  extreme  eastern  and  western  portions.  In  the 
central  portion  of  the  district  they  have  a  tendency  to  occur  in  groups, 
with  the  intervening  areas  showing  few  or  no  deposits.  The  largest  group 
of  mines  in  the  district,  and  for  that  matter  probably  the  largest  group  of 
iron-ore  deposits  of  such  quality  and  size  in  any  known  equivalent  area  in 
the  world,  is  in  the  vicinity  of  Hibbiug.  In  the  district  as  a  whole,  the 
deposits  are  less  numerous  and  less  in  bulk  adjacent  to  the  north  and  south 
boundary  of  the  iron-bearing  formation  than  in  the  middle  horizon,  though 
there  are  numerous  and  marked  exceptions  to  this  statement.  It  is  safe  to 
say  that  a  line  drawn  parallel  to  the  trend  of  the  district,  halfway  between 
the  north  and  south  boundaries,  would  cross  the  large  majority  of  the 
deposits.  Assuming  the  ore  to  occupy  about  one-foiirth  of  the  areas 
shaded,  the  proportion  of  the  area  of  the  ore  deposits  to  the  total  area  of 
the  iron-bearing  foi-rnation  of  the  district  is,  roughly,  5  per  cent,  and  for  the 
area  between  the  Hawkins  mine  and  Mesaba  station  the  iron  ore  occupies 
8  per  cent  of  the  area  of  the  iron  formation. 

While  no  iron  ore  has  been  found  in  the  few  holes  that  have  penetrated 
the  solid  A^irginia  slate  and  entered  the  underlying  formation,  it  is  not 
satisfactorily  proved  by  actual  drilling  that  iron-ore  deposits  do  not  occur 
in  this  part  of  the  iron-bearing  formation.     Iron  ores  have  been  found  in 

206 


THE  IRON-ORE  DEPOSITS.  207 

and  associated  with  slate  layers  in  the  iron-bearing-  formation  and  also  just 
under  the  edge  of  solid  black  Virginia  slate.  Yet  certain  facts  discussed 
in  connection  with  the  origin  of  the  ores  indicate  but  small  probability  of 
finding  ores  any  considerable  distance  under  the  Virginia  slate. 

The  western  portion  of  the  range,  west  of  Hawkins  mine,  shows  few 
deposits  and  these  are  of  comparatively  low  grade.  No  adequate  reason  is 
known  why  high-grade  ores  should  not  be  found  there  in  quantity,  but  the 
considerable  amount  of  drilling  thus  far  done  in  this  area  has  not  disclosed 
them.  Certainly  the  holes  put  down  in  this  part  of  the  range  have  disclosed 
far  less  high-gi"ade  ore  than  any  equivalent  number  of  holes  as  widely 
distributed  through  the  central  portion  of  the  range. 

The  iron  formation  in  the  eastern  part  of  the  Mesabi  cUstrict,  east  of 
Mesaba  station,  is  well  exposed  and  has  been  much  explored.  Indeed, 
considerable  exploration  was  done  here  before  the  central  portion  of  the 
range  was  discovered.  Yet  no  merchantable  deposits  of  ore  have  thus  far 
been  discovered.  It  is  thought  that  little  or  no  high-grade  ore  is  likely  to 
be  found  here  for  reasons  given  on  pages  272—274. 

SHAPE. 

The  shape  of  the  ore  deposits  can  not  be  better  described  than  by 
saying-  that  the  rock  bottoms  on  which  they  lie  form  shallow,  irregular 
basins,  usually  with  greater  horizontal  than  vertical  dimensions,  although 
rarely  the  reverse.  These  basins  have  all  the  complexity  and  relations  to 
other  troughs  of  ordinary  surface-drainage  basins  in  driftless  areas.  Indeed, 
topographic  maps  of  the  bottoms  of  most  of  the  Mesabi  iron-ore  deposits 
would  serve  fairly  well  as  maps  of  typical  surface-drainage  basins.  The 
basins  may  be  fairly  simple  with  only  minor  irregularities;  they  niay  slope 
gently  on  one  side  and  steeply  on  the  other;  there  may  be  overhanging 
shelves  on  one  or  both  banks;  there  may  be  salients  and  reentrants;  there 
ma,y  be  islands  of  rock;  there  may  be  irregularities  in  their  bottoms 
resulting  in  their  longitudinal  or  transverse  division;  there  may  be 
tributary  channels  coming  in  from  the  sides.  The  bottoms  of  the  basins 
seldom  show  gentle  slopes,  but  are  terraced,  the  slopes  of  the  ten-aces 
corresponding  to  the  dip  of  the  iron-formation  strata.  Usually  the  rock 
basins  containing  the  ore  have  a  very  considerable  pitch;  the  difference  in 
elevation  of  the  two  ends  may  be  as  much  as  100  feet.     The  lower  portions 


208  THE  MESABI  IRON-BEARING  DISTRICT. 

of  the  basins  containing  ore  are  commonly  below  the  level  of  the  outlets. 
The  upper  sides  of  the  ore  deposits  are  usually  irregular  and  perhaps  slightly 
below  the  level  of  the  surrounding  rock  surfaces.  These  features  are  due 
to  diffei'ential  erosion  and  to  the  scraping  of  the  glaciers  which  have  passed 
over  them  Because  of  this  fact  and  because  also  of  the  irregular  covering 
of  glacial  drift  the  pits  of  the  rock  basin  in  which  the  ore  deposit  lies  may 
not  be  apparent  at  the  surface.  Finally,  the  longer  directions  of  the  ore 
basins  may  be  either  parallel  or  transverse  to  the  ti'end  of  the  range. 

A  topogi-aphic  map  of  the  bottom  of  a  typical,  somewhat  irregular 
Mesabi  iron-ore  deposit,  the  Adams,  is  represented  in  PI.  XXIII. 

sizi:. 

The  Mesabi  iron-ore  deposits  have  horizontal  dimensions  varying-  from 
a  few  feet  to  almost  2  miles.  Commonly,  they  show  considerable  extension 
in  one  direction.  The  widths  seldom  exceed  a  quarter  of  a  mile,  while  the 
lengths  are  not  uncommonly  a  half  mile  or  more. 

The  thickness  rarely  reaches  350  feet  and  is  commonly  less  than  200 
feet.  Many  deposits  not  over  60  feet  >n  thickness  are  being  mined.  The 
maximum  depths  of  the  workings  of  some  of  the  mines  are  given  in  the 
following  table: 

Maximum  depth  of  mines  in  Mesabi  district. 

[June.  1902.] 

Feet. 

Adams 20.5 

Burt 1 20 

Duluth 97 

Biwabik 110 

Mahoning 7.5 

Sparta 12.5 

Malta •. 110 

Pillsbury 73 

Spruce 211 

Stevenson 40 

Hull 176 

Rust 192 

Sellers 13S 

Auliurn 218 

(ienoa 174 

Fayal UiS 

Mountain  Iron 150 


THE  IRON-ORE  DEPOSITS.  209 


KINDS   OF   ORE. 


The  Mesabi  iron  ores  are  for  the  most  part  shghtly  hydrated  hematites 
with  an  average  of  3  per  cent  of  combined  water  shown  by  cargo  analyses. 
It  is  likely  that  the  true  percentage  of  combined  water  may  be  somewhat 
larger  than  this,  for  before  it  is  measured  the  ore  is  dried  at  212°  F.,  a 
temperature  sufficient  to  drive  off  some  of  the  combined  water.  Asso- 
ciated with  the  slightly  hydrated  ores  are  abundant  layers  of  ores  of 
varying  thickness  differing  from  hematites  only  in  their  higher  percentage 
of  combined  water.  They  include  turg-ite,  2  Fe203  (94.7  per  cent),  HjO 
(5.3  per  cent) ;  goethite,  Fe203  (89.9  per  cent),  HjO  (10.1  per  cent) ;  limonite, 
2  FeaOs  (85.5  per  cent),  3  HjO  (14.5  per  cent);  and  possibly  even  xantho- 
siderite  Fe203  (81.6  per  cent),  2  H2O  (18.4  per  cent).  Magnetite  is  present, 
but  ver}^  sparsely,  in  the  ore  deposits. 

The  slightly  hydrated  hematites  are  ustially  dull  blue-black  or  brown 
earthy  varieties.  The  hard  blue  crystalline  hematites  are  rare  and  the  bril- 
liant specular  hematites  altogether  lacking.  The  more  hydrous  ores  are 
for  the  most  part  soft,  earthy  varieties.  Most  of  them  are  locally  called 
yellow  ocher,  or  brown  ore,  but  they  include  also  hard  crystalline  varieties. 
Rarely  both  the  hematite  and  hydrous  ores  appear  in  stalactitic  or  botry- 
oidal  forms  which  are  likely  to  be  observed  bordering  cavities  in  the  ore. 
Under  the  microscope  the  hematites  appear  oidy  as  dull-red  and  black 
opaque  aggregates,  and  the  hydrous  ores  as  dull-yellow  opaque  aggregates, 
but  when  examined  in  hand  specimens  they  are  seen  frequently  to  be 
made  up  of  small  ellipsoidal  granules  identical  in  size  and  shape  to  the 
greeualite  granules  of  the  greenalite  rocks  described  on  pages  101-115. 
The  magnetite,  when  present,  is  black  and  crystalline,  occurring  typically 
in  octahedra. 

East  of  the  Iron  Range  track,  through  most  of  range  14,  and  all  of 
ranges  13  and  12,  the  oxide  is  largely  magnetite,  which  has  not  been  segre- 
gated into  deposits  of  sufficient  extent  to  warrant  exploitation.  Westward 
from  Mesabi  station  the  magnetite  gives  way  rapidly  to  more  or  less 
hydrous  hematite,  and  through  the  central  and  western  portions  of  the 
range  is  found  in  but  small  quantity. 

Slightly  hydrous  hematites  and  the  more  hydrous  ores  are  almost 
everywhere  interbanded  in  thin  or  thick  layers,  yet  in  most  deposits  con- 
MON  XLIII — 03 14 


210 


THE  MESABI  IRON-BEARING  DISTRICT. 


siderable  zones  are  composed  predominantly  of  one  or  the  other  class  of 
ores.  The  hydi-ous  ores  are  more  abundant  near  the  tops  of  the  deposits, 
as  in  the  Mahoning,  Sharon,  Clark,  Oliver,  Grenoa,  Sparta,  Elba,  Commo- 
dore, Biwabik,  Duluth,  and  Hale  mines.  To  less  extent  they  appear  in 
middle  and  lower  horizons  in  the  deposits. 


JNniSrERAIiS  AND  ROCKS  CONTAINED  IN  THE  ORE. 

The  principal  mineral  constituent  associated  with  the  ore  is  chert  or 
quartz.  Average  cargo  analyses  of  the  ores  show  about  4  per  cent  of  silica, 
but  locally  the  percentage  of  silica  in  the  ores  runs  much  higher.  There 
may  be  found  all  stages  in  the  gradation,  from  ore  with  a  low  percentage 
of  chert  to  ferruginous  chert  with  only  a  low  percentage  of  iron.  Ferru- 
o'inous  chert  forms  the  wall  rocks  of  the  deposit,  occurs  as  pillars,  horses, 
or  shelves  projecting  from  the  bottoms  or  sides,  and  occurs  as  small  or  large 
masses  entirely  included  in  the  ore.  Even  where  the  percentage  of  chert 
is  as  low  as  4  per  cent  the  substance  may  be  observed  in  minute  grains 
with  the  microscope. 

Rarely  a  layer  of  the  feiTuginous  chert  in  a  deposit  may  be  so  disin- 
tegrated that  it  is  a  soft,  light-yellow  powder  resembling  fine  sand  or  tripoli 
powder.  The  particles  are  entirely  angular.  Analyses  published  by  SpuiT 
give  the  following  results: 

Analyses  of  silica  j>owde7\'^ 


Constituent. 

Per  cent. 

Fer  cent. 

SiO,                                                                                                                      

77.89 

13.55 

1.83 

.36 

Trace. 

.58 

.84 

1        4.45 

98.17 

ALO,                                                                                                                     

.50 

Fe,0,                                                                                         

1.03 

MgO                                                                     

Trace. 

CaO        .  .                                         

Trace. 

NajO 

.25 

K,0 

Trace. 

H2O-                                                                                     

H.,0+                                                      - 

Loss  on  itniition 

.19 

99.50 

100. 14 

"Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  10,  pp.  si  nnd  211. 


THE  IRON-ORE  DEPOSITS.  211 

Other  similar  layers,  somewhat  coarser,  are  found  on  examination  to 
consist  of  waterworn  sand  (to  be  discriminated  from  the  "sandy"  ores 
described  in  the  subsequent  paragTaph).  Some  of  the  sand  layers  are  cer- 
tainly the  result  of  washing  in  of  glacial  sand  along  cracks  from  above,  as 
the  layers  have  been  connected  with  the  surface  and  consist  of  fragments 
of  all  the  minerals  found  in  the  drift,  but  others  may  represent  a  disinte- 
gration of  original  sandy  layers  in  the  iron  formation. 

In  the  western  end  of  the  rang-e  some  of  the  iron  ores  contain  so  much 
disseminated  "sand"  that  up  to  1902  they  were  considered  unfit  for  use. 
The  "sand"  is  uniformly  disseminated  tln-ough  the  ore,  or  occurs  as  more 
or  less  iron-stained  layers.  Under  the  microscope  the  "sand"  is  seen 
to  consist  not  of  water-rolled  particles,  but  of  subangular  fragments 
of  chert  derived  from  the  disintegration  of  the  ferruginous  chert.  All 
stages  of  the  disintegration  may  be  observed  from  a  typical  ferruginous 
chert,  in  which  the  former  existence  of  greenalite  granules  is  indicated  by 
'the  distribution  of  chert  and  iron  oxide,  to  the  loose  particles  of  chert  which 
to  the  naked  eye  look  like  sand.  In  the  less  disintegrated  phases  the 
polygonal  and  angular  particles  of  chert  may  be  seen  to  be  separated  by 
thin  films  of  iron  oxide,  showing  the  cementation  of  the  particles  to  be  very 
weak.  The  disintegrated  chert  may  be  separated  from  the  ore  by  washing, 
but  whether  or  not  this  can  be  done  successfully  on  a  commercial  basis  is  a 
question  not  yet  satisfactorily  decided.  Experiments  thus  far  conducted, 
while  not  decisive,  indicate  that  it  can  be  done. 

A  small  quantity  of  griinerite  and  actinolite  in  columnar  forms  or  in 
radial  sheaves  may  occasionally  be  seen  associated  with  the  chert  and  ore, 
especially  in  the  ore  containing  considerable  magnetite. 

Crystals  of  calcite,  siderite,  dolomite,  quartz,  adularia,  pyrite,  mica, 
pyrolusite,  and  many  other  minerals  are  common  in  vuggs. 

It  is  not  known  in  what  mineral  form  the  phosphorus  occurs  in  the 
ores,  although  it  will  be  shown  on  a  subsequent  page  that  it  occurs 
probably  in  combination  with  alumina. 

In  the  Michigan  ranges  Prof  A.  E.  Seaman,  of  the  Michigan  School  of 
Mines,  has  determined  the  phosphorus  to  occur  largely  in  the  form  of 
apatite.  Apatite  has  been  searched  for  in  the  Mesabi  ores,  but  has  not 
been  found  in  clearly  identifiable  crystals.     Other  phosphorus  minerals, 


212  THE  MESABI  IRON-BEARING  DISTRICT. 

such  as  viviauite,  wavellite,  waguerite,  aud  collophanite,  have  been  looked 
for,  but  without  success. 

Layers  of  paint  rock,  ranging  in  thickness  from  a  fraction  of  an  inch 
to  several  feet,  occur  in  almost  every  deposit.  Kaolin,  or  clay  layers, 
either  white  or  yellow,  and  not  sufficiently  discolored  by  iron  to  warrant 
the  name  "paint  rock,"  are  also  occasionally  to  be  seen. 

Ferruginous  slate,  differing  from  the  normal  ores  only  in  containing 
a  higher  percentage  of  alumina,  forms  layers  in  the  ore  deposits. 

Occasionally  a  small  mass  of  bluish-black  ore  is  encountered  which 
nms  high  in  manganese  in  the  form  of  pyrolusite. 

Vein  quartz,  usually  much  brecciated,  is  a  common  feature  in  the 
ore  deposits.  It  usually  follows  irreg'ular  joints  and  fault  zones,  and 
occasionally  follows  the  bedding  for  a  considerable  distance.  The  breccia- 
tion  is  direct  evidence  of  considerable  movement  in  the  deposit  subsequent 
to  the  inti'oduction  of  quartz. 

Iron  pyrites  is  rarely  to  be  observed  in  quantity;  it  is  known  to  be 
sufficiently  abundant  to  lower  the  value  of  the  ore  only  along  the  edge  of 
two  deposits  on  the  range. 

Still  other  rare  rocks  or  minerals  in  the  ore  deposits  could  be 
mentioned.  In  the  Fayal  mine  is  a  peculiar  bluish  rock  with  a  greasy 
feeling,  showing  slickensides.  It  is  a  rock  rich  in  magnesium  and  colored 
by  ferrous  iron.     Its  oi'igin  is  not  known,  nor  is  it  important. 

CHEMISTRY. 

The  following  statements  concerning  the  composition  of  the  Mesabi 
ores  are  based  on  official  cargo  analyses,  as  published  by  the  American  Iron 
and  Steel  Association,  on  a  great  number  of  detailed  figures  furnished  by 
mining  superintendents,  engineers,  and  chemists  on  the  Mesabi  range,  and 
finally  on  general  statements  made  by  those  best  qualified  to  make  them. 
Among  those  who  have  given  especially  full  information  on  this  subject 
should  be  mentioned  the  Lerch  brothers,  chemists,  of  Hibbing,  Virginia,  and 
Biwabik;  Mr.  R.  B.  Green,  chemist  ofthe  Minnesota  Iron  Company;  Mr.  E.T. 
Griese,  chemist  ofthe  Duluth,  Missabe  and  Northern  docks  at  Duluth;  Mr. 
A.  P.  Silliman,  mining  engineer  aud  chemist,  Hibbing;  Mr.  A.  T.  Gordon, 
chemist.  Mountain  Iron;  Mr.  E.  J.  Johnson,  chemist  ofthe  Republic  group. 
To  mention  superintendents  and  mining  eiigineers  who  have  given  infor- 


THE  IRON-OEE  DEPOSITS.  213 

mation  would  be  to  give  a  list  of  those  connected  with  Mesabi  mining.  It 
must  not  be  understood  that  each  of  the  above-named  gentlemen  would 
agree  with  all  of  the  following  statements.  Indeed,  there  is  much  minor 
diversity  of  opinion. 

Average  cargo  analyses  of  Mesabi  ores  shipped  in  1901,  according  to 
figui'es  compiled  by  the  Iron  and  Steel  Association,  are  as  follows: 


214 


THE  MESABI  IRON-BEARING  DISTRICT. 


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THE  IRON-ORE  DEPOSITS. 


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218  THE  MESABI  IRON-BEARING  DISTRICT. 

The  aboA^e  fig-ures  show  that  the  Mesabi  ores  at  present  mined  contain 
a  hig-h  percentag-e  of  iron — indeed,  a  higher  percentage  than  is  shown  by 
the  average  of  all  the  ores  mined  in  the  other  ranges  of  the  Lake  Superior 
region.  At  the  present  time  ore  containing  less  than  58  per  cent  is  not 
mined  on  the  ilesabi,  except  in  small  quantity  for  mixing  with  higher 
grades,  thus  making  the  cargo  g-rade  above  58  per  cent. 

Tlie  slightly  hydrated  hematites  make  up  the  bulk  of  the  ore  shipped 
from  the  ]\Iesabi  range,  and  therefore  their  average  composition  is  approxi- 
mately that  given  above  for  the  entire  Mesabi  shipment. 

The  percentage  of  iron  in  the  yellow  or  brown  ores  averages  less  than 
the  tigures  given  for  the  hematites.  Fifty-six  to  60  per  cent  are  character- 
istic figures. 

The  loss  on  ignition,  above  shown,  is  presumabh'  largelv  combined 
water.  However,  this  may  not  represent  all  the  combined  water,  for  the 
ores  are  dried  at  212^  F.,  and  it  is  extremely  likel}'  that  at  this  temperature 
some  of  the  combined  water  is  driven  off.  Where  Mesabi  ore  has  been 
finel}"  powdered  and  dried  for  a  longer  time  than  usual  a  half  to  1  per 
cent  more  of  water  has  been  lost  by  this  drying."  Again,  there  mav  be  a 
really  greater  loss  of  combined  water  on  ignition  than  is  shown  by  the 
weight,  because  there  is  an  actual  gain  of  weight  due  to  the  oxidation  of  a 
ferrous  oxide  to  a  ferric  oxide.  On  the  other  liand,  the  loss  on  ignition 
may  be  partly  due  to  the  burning-  of  organic  matter,  or  to  the  conversion 
of  a  carbonate  to  an  oxide,  or  to  the  decomposition  of  a  sulphide  whereliy 
the  sulphur  is  eliminated.  But  while  the  average  of  3  per  cent  is  perhaps 
a  trifle  low,  the  figure  probably  represents  nearly  the  average  conditions. 
From  these  average  conditions  there  are  wide  variations,  for  it  is  known 
that  some  of  the  yellow  ores  are  highly  liydrated,  while  others  are  slightly 
so.  For  instance,  some  of  the  "j'ellow  ocher"  in  the  Biwabik  group  of 
mines  showed  an  average  content  of  water,  accordins'  to  H.  V.  Winchell, 
of  10.1  per  cent,  thus  being  goethite. 

The  wide  variation  in  moisture  driven  off  at  212° — that  is,  hygroscopic 
water — is  due  to  the  character  of  the  or  and  to  local  conditions.  A  porous 
ore  is  likely  to  contain  more  free  water  than  a  dense  ore.  An  ore  which 
has  been  standing  in  water  is  likely  to  contain  more  free  water  than  ores 
which  have  been  in  drier  places.     In  the  Mountain  Ii-ou  deposit  the  content 

'■'As  iH-r  letter  (if  R.  B.  Green,  chemist,  Minnesota  Iron  Co..  dateil  'March  4.  1902. 


THE  IRON-ORE  DEPOSITS.  219 

of  free  water  varies  fi'om  6  per  cent  at  the  top  of  tlie  open  cut  to  15  per 
cent  at  the  bottom,  and  all  is  above  ground  water.  When  mines  are  first 
opened  up  the  content  of  water  is  usually  larger  than  later  when  the  mine 
has  been  partially  drained.  A  heavy  rainstorm  also  will  make  a  difference 
of  2  or  3  per  cent  in  the  total. 

Only  two  deposits  on  the  range  are  known  to  contain  sulphur  in 
injurious  amounts.  The  sulphur  is  present  in  the  form  of  iron  pyrites  and 
is  usually  confined  to  the  edges  of  the  deposits.  In  mining  these  parts  of 
the  deposits  are  simply  passed  by.  The  hydrous  ores  perhaps  run  a  little 
higher  in  sulphur  than  the  nonhydrous  ores. 

The  great  vai'iation  in  silica  is  due  to  the  fact  that  the  iron  ore  grades 
into  ferruginous  chert.  Rocks  can  be  obtained  showing  all  percentages  of 
iron  and  silica,  but  those  containing  a  sufficient  amount  of  iron  to  be 
ranked  as  ores  seldom  contain  over  8  per  cent  of  silica.  In  the  western 
portions  of  the  range  the  silica  content  is  on  an  average  higher  than  in  the 
central  portion,  and  in  some  deposits  is  so  high  as  to  run  the  percentage 
of  iron  down  below  the  salable  limit . 

The  variation  in  alumina  is  due  to  the  varying  content  of  clayey  and 
slaty  material.  In  general  the  alumina  is  a  trifle  higher  in  the  yellow  ores 
than  in  the  blue  or  black  ores,  although  there  are  exceptions.  This  appears 
in  comparing  the  higher  and  lower  grades  of  ore  in  the  preceding  table. 
Throughout  the  district  the  lower  grades  of  ore  are  the  ones  which  are 
likeh''  to  contain  more  of  the  yellow  ore  than  the  remainder  of  the  ores. 
For  instance,  in  comparing  the  Mountain,  Oliver,  Juniata,  and  Preble 
grades  of  ore  from  the  Mountain  Iron  mine  it  appears  that  as  the  iron  runs 
down  the  alumina  runs  up.  The  paint  rocks  uniformly  contain  a  much 
higher  percentage  of  alumina  than  any  other  rock  in  the  iron  formation. 

As  phosphorus  in  considerable  quantity  prevents  use  of  ores  for  the 
acid  Bessemer  process,  the  phosphorus  content  in  an  ore  is  of  the  greatest 
importance.  The  Bessemer  limit  is  vague,  but  is  commonly  placed  at  0.045 
to  0.050  in  phosphorus.  From  60  to  70  per  cent  of  Mesabi  ores  would  be 
ranked  as  Bessemer.  However,  ores  containing  a  much  higher  percentage 
of  phosphorus  are  used  in  quantity  in  basic  open-hearth  furnaces,  and  with 
the  rapid  growth  of  the  open-hearth  method  of  steel  making  such  ores  will 
be  in  even  greater  demand. 

In  the  little  hydrated  hematites  the  phosphorus  is  below  0.05  per  cent, 


220  THE  MESABI  IRON-BEAKING  DISTRICT. 

although  occasionally  running  a  little  higher.  In  the  hydrous  oi'es  the 
average  of  the  phosphorus  is  higher.  The  common  figures  are  above  0.05 
per  cent.  It  is  not  necessary  to  give  detailed  figures  for  particular  ores. 
Chemists,  mining  engineers,  and  mine  managers  all  agree  to  this.  As  the 
hydrous  ores  are,  on  the  whole,  more  abundant  in  the  upper  portion  of  the 
deposits  than  elsewhere,  the  phosphorus  is  correspondingly  more  abundant 
at  this  horizon. 

It  will  be  seen  below  that  the  Mesabi  ore  is  partly  in  the  form  of  soft 
dirt  and  partly  in  hard  lumps.  To  ascertain  whether  or  not  there  is  any 
difi'erence  in  the  percentage  of  phosphorus  in  the  hard  and  soft  lumps,  in 
order  to  regulate  the  sampling,  Mr.  R.  B.  Grreen,  of  the  Minnesota  Iron 
Company,  made  a  considerable  number  of  analyses  from  cargoes  of  Canton, 
Norman,  and  Fayal  ores,  with  results  as  follows: 

Percentage  of  pho«2}horus  in  hard  and  soft  Iwinps  of  ores. 

Lump.  "  Fine. 

0.  046  0.  033 

.043  032 

.048 .053 

.045  048 

.059  067 

.072  059 

.064  050 

.041  037 

.047  036 

.  0517  (Average) 046 

These  figures  indicate  in  general  a  slightly  higher  percentage  of 
phosphorus  in  the  hard  lumps  than  in  the  soft  ones.  A  similar  result  was 
obtained  by  Mr.  E.  T.  Griese  in  comparing  the  hard  and  soft  lumps  of  the 
Biwabik  mine.  The  soft  material  ran  below  0.035,  while  the  harder  lumps 
ran  up  to  0.040  to  0.050. 

Ores  containing  over  IJ  jjer  cent  of  manganese  are  shipped  only  to  a 
small  extent.  However,  there  are  present  in  the  Mesabi  district  ores 
containing  a  considerable  higher  percentage  of  manganese.  In  the 
Mountain  Iron,  the  Moose,  and  the  Oliver  deposits  bunches  of  ore  have 
been  found  to  run  locally  from  15  to  61  per  cent  in  mang-anese.  The 
Oliver  property  is  the  only  one  on  the  range  containing  sufticient  amoimt 
of  manganese  to  prevent  the  shipment  of  any  considerable  proportion  of 
its  ore.     Attempts  have  been  made  to  utilize  such  ores  as  a  manganese  ore. 


THE  IRON-ORE  DEPOSITS. 


221 


but  thus  far  without  success.  In  the  samples  of  hard  and  soft  material 
analyzed  by  Mr.  Green  it  was  found  that  the  manganese  was  slightly  higher 
in  the  hard  lumps,  than  in  the  soft  ones.  A  similar  result  was  obtained  by 
Mr.  Grordon  of  the  Mountain  Iron  mine,  as  a  result  of  analyses  of  Mountain 
Iron  and  Oliver  ores.  In  the  Biwabik  mine  the  hard  ore  there  found  runs 
distinctly  higher  in  manganese  than  the  soft  ore.  Finally,  certain  of  the 
yellow  ores  run  as  high  as  1  per  cent  of  manganese,  while  the  blue  and 
black  ores  seldom  go  over  0.50  per  cent,  except  when  close  to  the  wall  rock. 
Analyses  of  magnetite  from  the  eastward  continuation  of  the  Mesabi, 
in  the  neighborhood  of  Akeley  and  Gunflint  lakes,  are  as  follows: 


Analyses  of  magnetite  from  neighborhood  of  Akeley  and  Gunflint  lakes. 


Constituent. 

1. 

2, 

3. 

4. 

Fe          - 

58.40 

54.01 

63.98 

61.95 

Fe,Oi 

85.55 

SiOj 

8.22 
.52 

9.37 
.07 

8.90 

11.39 

ALO,                                       -                     

Trace. 

CaO                                                      

.22 

MeO 

3.44 

P 

.36 

4.92 

None. 

.32 
5.02 

None. 

.28 
None. 
Trace. 

.02 

Mn                                          

Ti                                     

None. 

s 

Trace. 

1,  2,  and  3.  (Average  of  6  samples).  From  NE.  \  of  NE.  \  sec.  29,  T.  65  N.,  E.  4  W.  (west  end 
of  Gunflint  Lake).  Analysis  by  Rattle  and  Nye,  Cleveland.  Published  by  N.  H.  Winchell,  Geol. 
Nat.  Hist.  Survey  Minnesota,  Bull.  No.  6. 

4.  From  SE.  \  sec.  30,  T.  62  N.,  R.  10  W.  (transported  masses  of  gabbro).  Analysis  by  C.  F. 
Sidener.     Published  by  N.  H.  Winchell,  Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  6. 


222 


THE  MESABI  IRON-BEARING  DISTRICT. 


The  magiietites  at  the  east  end  of  the  range  have  not  yet  been  found 

in  pajang  quantities.     Analyses  of  magnetite  from  ranges  12  and  13  are  as 

follows : 

Analyses  of  magnetite  from  ranges  12  and  13. 


Constituent. 

1. 

2. 

SiO„                   

1.16 

1.81 

69.08 

11.89 

ALO,                                

.34 

Fe,0,                            - 

Fe.Oi                        

87.00 

FeO                                                                         

27.10 

69.43 

.25 

0.53 

None. 

.06 

Fe 

63.07 

MgO                                            

.80 

CaO                                             .                

0.20 

TiOj                                              

None. 

p,0-                         

p              

.056 

MnO      

.33 

S  

Trace. 

1.  From  the  east  end  of  Mesabi,  SE.  i  of  SE.  J  sec.  34,  T.  61  N.,  E.  12  W.     Analysis  by  W.  H. 
Melville,  for  W.  S.  Bayley. 

2.  From  the  NE.  \  of  sec.  23,  T.  60  N.,  R.  13  W.  (near  Iron  Lake).     Analysis  by  C.  F.  Sidener. 
Published  by  N.  H.  Winchell,  Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  6. 


THE  IRON-ORE  DEPOSITS. 


223 


It  will  be  noted  that  the  magnetites  contain  little  or  no  titanium.  In 
this  they  differ  from  the  magnetites  occurring-  in  the  gabbro.  Analysis  of 
the  latter  are  as  follows: 

Analyses  of  magnetites  in  gahhro. 


Constituent. 

1. 

2. 

3. 

SiOj 

20.90 
1.75 

Trace. 
2.63 
2.23 

None. 

2.02 

2.68 

Trace. 

11.37 

A1,0, 

1.32 

CaO 

.10 

MgO 

2.73 

TiO,  

16.03 

P 

.01 

P,0, 

.03 

FeO 

2.01 
70.29 
52.46 

14. 42 

Fe,0^                                                                       .  . 

80.78 

58.  48 

12.09 

2.40 

53.33 

Fe  .                                                                                            .                   . 

49.40 

Ti  .                                                             .                           

CrjO..                                             

S  ..                                                      

Trace. 

1.  From  the  neighborhood  of  Iron  Lake  in  T.  65  N.,  R.  3  W.     Analysis  by  Prof.  J.  A.  Dodge" 
Published  by  N.  H.  Winchell,  Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  6. 

2.  From  SE.  I  sec.  36,  T.  65  N.,  E.  3  W.  (Iron  Lake).     Analysis  by  E.  S.  Eobertson,  Pittsburg. 
Published  by  N.  H.  Winchell,  Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  6. 

3.  From  sec.  36,  T.  63  N.,  E.  10  W.     Analysis  by  C.  F.  Sidener.     Published  by  N.  H.  Winchell, 
Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  6. 

The  paint  rock  frequently  associated  with  ore  deposits  has  a  imich 
lower  average  content  of  iron  than  either  the  hematite  or  hydrous  ores. 
It  averages  all  the  way  from  12  to  45  per  cent  or  even  to  55  per  cent. 
Figures  between  40  and  50  per  cent  are  the  most  common  ones.  Phos- 
phorus is  also  usually  but  not  invariably  high.  The  usual  range  is  from 
0.070  to  0.150.  Alumina  is  also  much  higher  in  the  paint  rock  than  in  the 
hematite  or  hydrous  ores.  The  figures  run  as  high  as  7  per  cent  (Lerch): 
3  to  4  per  cent  (Griese)  may  be  the  average.  The  water  content  is  char- 
acteristically large. 

TEXTURE  AND   STEUCTUEE. 

The  Mesabi  ores  range  in  texture  from  large  crystalline  masses 
requiring  the  use  of  a  crusher,  as  at  Biwabik,  to  fine,  soft  dirt,  which  runs 
like  dust  between  the  fingers.  In  general  the  ores  at  Virginia  and  Eveleth 
and  eastward  are  somewhat  harder  and  coarser  than  those  of  Mountain 


224 


THE  MESABI  IRON -BEARING  DISTRICT. 


Iron,  Hibbing,  and  vicinity,  although  there  are  exceptions  both  ways. 
The  deposits  are  thin,  bedded  in  layers  varying  from  a  fraction  of  an  inch 
to  several  inches,  or  even  feet.  Usually  some  of  the  layers  are  soft  and 
pulverulent  while  others  are  more  or  less  hard  and  broken  up  into  small 
parallelopiped  blocks  by  cross  fractures.  The  best  physical  texture  for 
furnace  purposes,  a  medium  hard  ore  in  small  lumps,  is  frequently  found 
along  the  edges  of  the  deposits.  A  stroke  of  the  pick  at  almost  any  point 
in  a  deposit  will  loosen  a  mass  of  soft  ore  mixed  with  small  blocks  of  hard 
ore  which  seldom  exceed  a  few  inches  in  length  and  breadth.  The  average 
texture  of  some  of  the  ores  is  shown  by  the  following  screening  figures  of 
representative  ores  kindly  furnished  by  Mr.  R.  B.  Green,  chemist  of  the 
Minuesota  Iron  Company: 

Average  texture  of  ore. 


Ore  not  pass- 
ing through 
a  screen  with 
8  meshes  to 
the  inch,  a 


Ore  passing 

through 

screen  with 

S  meshes  to 

the  inch. 


Ore  passing 

through 

screen  with 

20  meshes  to 

the  inch. 


Ore  passing 

through 

screen  with 

100  meshes  to 

the  inch. 


1898. 
Audrey  ore,  Auburn  mine  (41  cargoes)  . 
Auburn  ore,  Auburn  mine  (13  cargoes). 

Fayal  ore  (43  cargoes) 

Genoa  ore  (34  cargoes) 

Sparta  ore  (44  cargoes) 

Elba  ore  (6  cargoes) 

1901. 

Norman  ore  (23  cargoes) 

Top  Brown,  Sparta  mine  (8  cargoes)  . . . 
Vulcan  ore,  Spruce  mine  (18  cargoes)  . . 


6.79 
6.92 
5.85 
Mountain  Iron  ore ;      75  to  80  '     75  to  80 


Per  cent. 
60.88 
51.73 
72.17 
71.47 
65.71 
64.13 

59.38 
64.92 
65.92 


Per  cent. 
9.52 
7.96 
8.42 
7.09 
5.58 
6.96 


Per  cent. 

Per  cent. 

18.72 

10.88 

20. 38 

19.92 

15.93 

3.48 

18.42 

3.01 

15.  64 

13.09 

20.05 

8.80 

18.16 

16.42 

15.35 

75  to  80 


15.  66 

11.74 

12.88 

20  to  25 


«'  Determined  in  natural  state  after  taking  from  cars  and  drying. 

The  looseness  of  the  ore  as  it  lies  in  the  deposit  is  shown  by  the  fact 
that  in  computing  the  tonnage  of  Mesabi  ores  11 J  to  14,  or  rarely  even  17 
to  18,  cubic  feet  are  allowed  per  long  ton  (2,240  pounds),  whereas  in  the 
hard-ore  deposits  of  other  ranges  the  common  figures  are  8  or  9  cubic  feet. 

If  tlie  ore  were  all  limonite  with  a  specific  gravity  of  3.6,  and  if  a  ton 
be  supposed  to  occupy  14  cubic  feet,  the  calculated  pore  space  would  be  29 
per  cent.  If  the  ore  were  all  hematite  with  a  specific  gravity  approaching 
5,  tlie  pore  space  would  be  48  per  cent.     If  a  ton  of  ore  be  supi)osed  to 


THE  IRON-ORE  DEPOSITS.  225 

occiii)j  11 J  cubic  feet,  there  would  be  13  per  cent  poi'e  space  if  it  were 
limonite  and  38  per  cent  if  it  Avere  hematite.  As  the  ore  is  an  intermediate 
variety  between  hematite  and  limonite,  it  is  probable  that  the  average  pore 
space  is  somewhere  in  the  neighborhood  of  35  per  cent,  though  locally 
showing  wide  variations.  In  contrast  to  this  the  pore  space  in  the  old  range 
ores  is  commonly  less  than  20  per  cent. 

The  bedded  structure  of  tlie  Mesabi  ore  deposits  is  a  striking  feature. 
A  single  bed  commonly  shows  much  persistence  in  color  and  texture  where 
followed  out  laterally,  but  differs  in  these  particulars  from  beds  above  and 
below,  with  the  result  that  the  bedding  structure  is  made  most  conspicuous. 
(See  Pis.  XXIV  to  XXXIII.)  White  and  colored  efflorescence  along  the 
bedding  still  further  emphasizes  it.  The  bedding  planes  in  general  pitch 
gently,  perhaps  8°  to  20°,  toward  the  lower  end  of  the  basin  in  which  they 
lie.  The  beds  may  also  dip  gently  from  the  side  of  the  channel  in  toward 
its  axis,  as  in  the  Oliver  mine,  or  may  have  a  monoclinal  tilt,  as  in  the  Hale 
and  Kanawha  mines.  (See  PI.  XXXIII.)  In  exceptional  instances  dips  are 
as  high  as  50°  to  60°,  as  at  the  Stephenson,  Sauntry-Alpena,  Sparta,  and 
Kanawha  mines.  Close  to  the  wall  rocks  of  the  deposits  the  dip  of  the 
beds  of  ore  usually  becomes  suddenly  stee^jer  and  may  jump  up  45°  or 
more  at  the  immediate  contact.  Besides  having  the  above  general  attitudes 
the  iron-formation  layers  are  much  contorted  in  a  minor  way.  In  walking 
through  any  of  the  open  pits  of  the  mines  the  gentle  minor  undulations  of 
the  layers  are  everywhere  apparent,  and  here  and  there  folds  of  unusual 
sharpness  stand  out  conspicuously. 

Some  of  the  dips  observed  in  the  Mountain  Iron  mine  are  as  follows: 

Dips  observed  in  Mountain  Iron  mine. 

AT  LOWEST  LEVEL  ON  EAST  SIDE  OF  PIT  GOING  SOUTH. 


8°S. 

30°  SSW. 

8°  S. 

20°  N. 

1°S. 

6°  S. 

5°  NW. 

2°  E. 

5°S. 

4°N. 

10°  NNE. 

0°. 

10°  ssw. 

4°N. 

6°S. 

11°  SE. 

7°SE. 

9°  N.      • 

22°  S. 

3°S. 

8°  S. 

35°  N. 

AT  SECOND 

LEVEL  ON  EAST  SIDE  OF  PIT  GOING  NORTH. 

19°  SE. 

40°  SE. 

6°  NW. 

20°  SW. 

3°  NW. 

6°  E. 

0°. 

8°  SE. 

5°  NNW. 

39°  SSW. 

6°NW. 

15°  W. 

11°  S. 

44°  SSW. 

5°  NW. 

20°  W. 

20°  SE. 

4°E. 
MON   XLIII — 03 

-15 

7°SW. 

226  THE  MESABI  IRON-BEARING  DISTRICT.      ^ 

Dips  in  tiie  Oliver  mine  at  Virginia  are  as  follows: 

Dij>s  in  Oliver  mine. 

GOING  NOETH  TO  MIDDLE  OF  PIT. 

9°  NW.  7°  NW. 

ACROSS  SY^'CLINE  AT  UPPER  END  OF  PIT  FROM  SOUTH  TO  NORTH. 

21°  NE.  9°  NE.  19°  SW. 

GOING  WEST  ALONG  NORTH  WALL. 

12°  SSW.  30°  SSW.  15°  W.  40°  SSW. 

35°  SSW.  20°  SSW. 

Dips  in  the  Biwabik  mine,  begimiing  at  east  end  of  mine  and  running 
along  the  north  wall  on  the  upper  level,  are  as  follows: 

Dijjs  in  Bvwabih  mine. 


28°  S. 

15°  W. 

12°  S. 

14°  S. 

18°  SSW. 

13°  SSW. 

16°  S. 

12°  S. 

19°  WSW. 

14°  S. 

17°  S. 

19°  SW. 

20°  S. 

20°  SSW. 

The  bedding,  wliile  in  general  roughly  parallel  to  the  surface,  in  some 
deposits  shows  discordance  with  the  surface.  In  the  Sauntiy  mine,  for 
instance,  the  surface  of  the  deposit  may  be  observed  to  cut  diagonally 
across  the  layers  of  the  ore  deposit. 

Nodules  of  iron  ore  are  frequently  found  with  their  gi-eater  diameters 
parallel  to  the  bedding.  The  nodules  vary  fi'om  a  fraction  of  an  inch  up 
to  6  or  8  inches  or  more  in  size.  They  are  frequently  hollow  and  some- 
times show  concentric  arrangement  of  hydrated  and  nonhydrated  ores. 

In  addition  to  the  bedded  and  folded  structures,  the  ore  deposits  show 
many  fractures,  especially  along  the  contact  with  the  wall  rock.  The 
individual  layers,  where  hard  enough,  are  broken  into  small  blocks  by 
fractures  which  in  the  main  are  independent  of  those  in  the  layers  above 
and  below.  Also  numerous  joints  and  faults  cut  across  the  ore  deposit. 
Indeed,  in  some  deposits  there  is  scarcely  a  cubic  yard  which  is  not 
crossed  by  one  or  more  such  fractures.  For  the  most  part  there  is  little 
displacement  along  fractures,  and  where  displacement  does  occur  it  is 
measured  by  inches  rather  than  feet.  Rarely  a  fault  of  several  feet  may 
l)c  ()l)sci-vc(l,  and  this  is  likely  to  be  near  the  contact  of  tlie  ore  witli  tlie 
wall  rock. 


THE  IRON-ORE  DEPOSITS.  227 

The  numerous  joints  and  faults  suggest  that  the  apparent  gentle  folds 
in  the  iron-bearing  strata  are  not  diie  entirely  to  actual  bending  of  the 
strata^  but  are  due  in  part  to  minute  displacement  along  the  closely  spaced 
fractures. 

THE  ROCKS   FOKMIKG  THE   BOTTOMS  ATsTD   SIDES   OF  THE   ORE 

DEPOSITS. 

The  rocks  adjacent  to  the  Mesabi  iron-bearing  deposits  are  ferruginous 
cherts,  more  particularly  the  altered  varieties,  ferruginous  slates,  paint 
rocks,  and  even  unaltered  slates.  In  abundance  the  rocks  stand  in  the 
oi'der  named.  In  one  mine,  the  Hale,  the  north  wall  of  the  ore  deposit  is 
a  micaceous  phase  of  the  Pokegama  quartzite  associated  with  a  schistose 
Archean  basalt.  The  north  wall  of  the  Biwabik  mine  also  may  be  quartzite, 
although  exploitation  has  not  yet  shown  this.  The  conditions  in  these  two 
mines  are  exceptional. 

STRITCTURAIL,  RELATIOlSrS  OF  THE  ORES  TO  THE  ADJACENT  ROCKS. 

The  shape  of  the  ore  deposits  and  the  attitude  of  then-  layers  can  be 
easily  made  out  from  the  mining  and  exploration  work  done  on  them.  But 
it  is  difficult  to  make  positive  statements  as  to  the  structural  relations  of  the 
Mesabi  iron  ores  to  the  adjacent  rocks  for  the  reason  that  a  very  small 
proportion  of  the  deposits  of  the  range  have  been  exploited  to  a  sufficient 
extent  to  show  such  relations.  Many  deposits  have  not  been  opened  up  at 
all,  and  the  preliminary  exploration  does  not  reveal  information  of  this  sort. 
In  the  underground  mining  the  di'ifts  reach  the  wall  rock  in  too  few  places 
to  warrant  general  statements  as  to  the  structure.  Most  of  the  open  cuts 
also  have  as  yet  either  not  reached  the  wall  rock  at  all  or  have  reached 
it  only  on  one  side.  Assembling  all  the  data  available  concerning  the 
relations  of  the  ore  to  the  wall  rock,  the  following  general  statements  seem 
to  be  warranted: 

(1)  In  a  few  mines,  as  for  instance  the  Oliver,  the  layers  of  rock 
adjacent  to  the  ore  have  such  attitudes  as  to  show  the  ore  to  lie  near  the 
axis  of  a  gently  pitching  trough  formed  by  the  folding  of  the  rock  layers. 

The  ore  deposits  of  this  class  have  relations,  which  are  expressed  in 
fig.  9.  On  each  side  the  wall  rocks  dip  in  toward  the  ore  deposit.  The 
upper  layers  abut  against  the  ore  deposit.  The  lower  layers  form  a  gentle 
trough  beneath,  which  usually  has  many  minor  undulations.     The  iron-ore 


228 


THE  MESABI  IRON-BEARING  DISTRICT. 


beds  themselves  are  bowed  iuto  a  gentle  trough  more  or  less  complex,  with 
slopes  similar  to  those  of"  the  rock  layers  below  and  at  the  sides.  In  other 
words,  the  layers  of  the  ore  deposit  and  the  layers  of  the  rock  together  make 
up  a  g-entle  syncline,  with  even  slopes.  This  does  not  mean  that  the  slopes 
of  the  bottom  of  the  deposit  are  gentle  and  even.  Indeed,  the  sm-face  of 
the  contact  of  the  rock  and  ore  is  exceedingly  irregular,  as  ah-eady  noted, 
and  usually  terraced,  and  the  dip  of  the  contact  sui-face  is  usually  steeper 
than  the  dip  of  the  strata.  In  other  words,  the  bottom  of  the  deposit 
forms  a  trough  with  much  more  in-egular  bottom  and  much  steeper  slopes 
at  the  sides  than  the  trough  formed  by  the  bowing  of  the  sti'ata  of  the  iron 
formation  and  ore  deposit  together. 


a^^Slat^ 


Iron  ofe 


Fig.  9. — Ideal  cross  section  of  a  Mesabi  ore  deposit  showing  relations  to  ferruginous  chert  and  impervious  slate  layers. 

The  longer  directions  of  the  ore  deposit  in  such  cases  are  usually 
parallel  to  the  axis  of  the  trough. 

(2)  The  rock  layers  about  other  ore  deposits  form  jjitching  troughs, 
but  the  folding  is  so  slight  that  the  pitch  is  far  more  conspicuous  than  the 
dip  of  the  limbs,  and  in  this  case  the  rock  layers  on  all  sides  of  the  deposit 
have  essentially  a  monoclinal  attitude.  The  longer  direction  of  the  deposit 
may  be  either  parallel  or  transverse  to  the  monoclinal  pitch,  but  it  is  usuallv 
transverse.  The  Canton,  Biwabik  (PI.  XXXII),  Cincinnati,  and  Saniitry 
(PI.  XXXI)  mines  are  instances  of  this. 

(3)  About  still  other  deposits  the  rock  layers  show  no  traces  of 
synclinal  flexure,  and  the  dip  is  essentially  monoclinal  with  minor 
variations  in  degree.  It  is  not  unlikely  in  some  cases  that  the  layers, 
while  essentially  monoclinal,  are  even  slightly  bowed  into  anticlines.  The 
deposit  ill  this  case  lies  ou   tlie  linib  of  one  of  tlic  gentle  folds  into  wliicli 


THE  IRON-ORE  DEPOSITS. 


229 


the  iron  formation  is  flexed,  either  the  great  major  fold  or  one  of  the  minor 
cross  folds  giving  the  gentle  tilting  of  the  iron  formation  to  the  south. 
Instances  of  such  relations  appear  at  Hale  fPl.  XXXIII),  Kanawha,  and 
Sparta  mines.  The  longer  direction  of  the  deposit  is  nearly  always 
transverse  to  the  pitch. 

A  generalized  cross  section  of  a  Mesabi  ore  deposit  parallel  to  the 
pitch  of  the  fold  or  parallel  to  the  dip  of  the  monoclinal  rock  strata  is  shown 
in  fig.  10. 

In  general  it  is  likely  that  more  of  the  ore  is  to  be  found  in  structural 
synclines  in  the  iron  formation  than  elsewhere,  but  this  may  not  be  in  all 
cases  apparent  in  the  attitude  of  the  layers  immediately  adjacent  to  the 
ore  deposit,  for  the  ore  deposit  may  fill  only  a  very  small  portion  of  a  broad 
and  gentle  syncline,  or  be  one  of  several  deposits  in  such  a  syncline,  and 


Fig.  10.— Ideal  section  parallel  to  the  pitch  oi  a  Mesabi  ore  deposit,  showing  relations  to  ferruginous  chert  and  to 
impervious  slate  layer. 

the  minor  structure  of  the  iron-formation  layers  immediately  adjacent  to 
the  ores  may  give  no  evidence  of  the  existence  of  the  major  syncline  in  the 
iron-formation  layei's.  These  structural  relations  are  explained  in  connec- 
tion with  the  discussion  of  the  origin  of  the  ores,  where  it  is  shown  that  the 
ores  have  been  concentrated  through  the  agency  of  underground  waters 
working  their  way  through  a  gently  folded  and  much  fractured  formation, 
in  which  the  major  flow  of  water  has  been  directed  by  the  broad  gentle 
synclines  in  the  iron  formation,  but  in  which  the  cross  fractures  have  locally 
concentrated  the  circulation  and  given  it  most  capricious  turns. 

The  structural  relations  of  the  ore  to  the  wall  rock  at  the  immediate 
contact  in  the  above  cases  may  be  any  of  the  following: 

(1)  The  layers  of  the  wall  rock  may  grade  into  the  layers  of  the  ore 
without  change  of  dip.  (PI.  XX,  fig.  B.)  This  is  likely  to  be  observed  at 
the  contact  of  the  ore  with  horses  of  rock  or  islands  of  rock  within  the  ore. 


230  THE  MESABI  IKON-BEARING  DISTRICT. 

(2)  The  layers  of  tlie  wall  rock  may  grade  into  the  la^'ers  of  the  ore, 
and  at  the  contact  there  is  a  sharp  downward  deflection  of  the  ore  layers. 
(PI.  XX,  fig.  A.)  The  layers  of  the  ore  may  be  almost  perpendicular,  and 
dips  of  45°  are  common.  Within  a  few  feet,  or  at  least  a  few  yards,  the 
layers  of  the  ore  deposit  take  on  their  usual  gentle  dip. 

(3)  Accompanying  the  downward  flexure  of  the  ore  layers  there  may 
be  jointing,  faulting,  or  brecciation.  Where  the  flexures  of  the  layers 
pass  into  fractures,  the  displacement  is  usually  small,  a  few  inches  or  at 
most  a  few  feet.  In  certain  mines  the  displacement  between  the  ore  and 
the  wall  rock  may  have  been  greater,  although  decisive  evidence  is 
lacking  The  steep  walls  to  be  seen  in  some  deposits,  as,  for  instance, 
the  Oliver,  Elba,  and  Canton,  have  sometimes  been  taken  as  evidence  of 
extensive  faulting.  While  the  walls  are  steep  as  compared  with  the 
average  of  the  wall-rock  slopes  on  the  range,  in  detail  they  are  terraced, 
some  of  the  terraces  running-  out  20  feet  or  more.  It  is  apparent  that 
there  has  not  been  any  great  displacement  parallel  to  the  wall,  for  the  ter- 
races would  have  required  a  tremendous  local  disturbance  of  the  ore  close 
to  the  contact  during  the  movement,  and  this  does  not  appear.  North  of 
the  Biwabik  mine  in  the  NW.  ^  of  NW.  i  sec.  2,  T.  58  N.,  R.  16  W., 
a  pit  has  been  sunk  70  feet  in  ore  just  60  feet  south  of  a  pit  showing 
Pokegama  quartzite.  If  the  quartzite  passes  under  the  ore,  it  must  do 
so  with  a  dip  of  over  45°,  a  dip  much  greater  than  the  average  in 
the  Upper  Huronian,  although  such  dips  are  indeed  to  be  observed. 
The  quartzite  is  much  broken  up  and  disintegrated  here,  and  it  is  not 
impossible  that  we  have  here  a  fault  with  a  considerable  displacement. 
At  the  Hale  and  Kanawha  mines  the  open  cuts  show  the  north  wall  to 
be  the  green  rock  of  the  Archean,  with  thin  films  of  micaceous  Pokegama 
quartzite  and  iron  formation  adhering  to  it.  The  vertical  and  lateral  dimen- 
sions of  the  deposit  parallel  to  the  wall  are  greater  than  that  normal  t<^  it, 
and  the  dip  of  the  iron  formation  and  ore  strata  is  steep,  being  in  some 
places  as  high  as  60°  to  the  south.  (See  PI.  XXXIII.)  The  attitude 
and  steep  dip  of  the  deposit  are  in  accord  with  the  idea  that  it  has  been 
developed  in  a  plane  of  weakness  along  tlie  contact  at  the  Archean  and 
Upper  Huronian,  where  there  has  been  considerable  movement,  but  there 
is  no  direct  evidence  of  this. 


PLATE  XX. 


231 


PLATE    XX. 

VIEWS    OF   THE    CONTACT    OF    ORE    WITH    WALL    ROCK    IN    THE    BIWABIK    AND   MOUNTAIN 

IRON    jVHNES. 

Fig.  -1. — This  view  is  taken  from  the  west  end  of  the  Biwablk  mine.  The  strata  on  the  left  side 
of  the  picture  are  ferruginous  chert,  and  those  on  the  right  side  are  ore.  The  fairly  sharp  downward 
bend  of  the  strata  from  left  to  right  near  the  center  of  the  field  is  at  the  contact  of  the  ore  and 
ferruginous  chert.  The  layers  of  the  ferruginous  chert  are,  for  the  most  part,  directly  continuous 
with,  and  grade  into,  the  layers  of  ore,  the  gradation  being  accomplished  within  a  few  feet.  The 
flexure  at  the  contact  is  due  to  the  slumping  down  of  the  ore  layers,  which  is  caused  by  abstraction 
of  silica  in  solution,  as  explained  on  page  262.  This  is  a  characteristic  feature  at  the  contact  of  the 
ore  and  wall  rock  in  most  deposits  on  the  range. 

Fig.  B. — This  view  shows  a  contact  of  the  ore  and  the  ferruginous  chert  forming  the  wall  rock  in 
the  south  railway  approach  of  the  Mountain  Iron  mine.  The  upper  strata,  appearing  lighter  colored 
and  more  coarsely  bedded  than  the  lower,  are  ferruginous  chert;  the  lower  flne-bedded  strata  are  iron 
ore.  The  rock  forms  a  thin  shelf  projecting  over  and  into  the  ore  deposit  at  this  point.  The  layers 
of  ore  at  the  south  end  of  the  mine  grade  directlj'  without  any  disturbance  into  layers  of  ferruginous 
chert  forming  the  wall  rock;  the  layers  of  ore  and  rock  are  strictly  parallel.  Near  the  contact  they 
become  interleaved,  layers  of  ore  extending  well  into  the  wall  rock,  and,  vice  versa,  layers  of  wall 
rock  extending  wel  into  the  ore. 

232 


U.    S.    GEOLOGICAL  SURVEY 


MONOGnAPH  XLin       PL.    XX 


A.     CONTACT  OF   ORE   WITH    WALL   ROCK    IN    BIWABIK    MINE. 


j:      CONTACT   OF   ORE   WITH    WALL    ROCK    IN    MOUNTAIN    IRON    MINE. 


THE  IRON-ORE  DEPOSITS.  233 

(4)  The  jointing,  faulting,  or  brecciation  may  appear  at  the  contact 
without  any  marked  change  in  dip  of  the  layers  on  either  side. 

A  single  deposit  may  show  part  or  all  of  these  relations  at  the  contact 
of  the  rock  and  ore.  Indeed,  it  is  rare  that  a  deposit  does  not  show  most  of 
them  at  different  places  along  the  contact.  However,  the  relations  described 
in  (2)  and  (3)  are  by  far  the  most  common.  In  any  case  the  ore  does  not 
run  into  the  rock  along  an  even  plane,  but  in  a  series  of  terraces,  as  ah-eady 
described.  Moreover,  the  contact  is  a  zigzag  one  in  plan.  The  contact  of 
the  ore  and  wall  rock  has  usually  followed  vertical  joints  which  intersect 
one  another  at  many  angles.  These  angularities  may  be  either  great  or 
small;  a  large  jDrojecting'  corner  may  carry  on  it  many  minor  corners,  and 
these  in  turn  small  ones  but  a  few  inches  in  size. 

PETKOGRAPHIC  RELATIONS  OF  THE  ORES  TO  THE  ADJACENT  ROCKS. 

Where  jointing,  faulting,  or  brecciation  does  not  prevent,  the  lateral 
transition  of  the  layers  of  the  wall  rock  into  the  layers  of  the  ore  can  be 
clearly  seen.  The  transition  is  sudden,  usually  being  completely  accom- 
plished within  a  fraction  of  an  inch,  but  requiring  in  jjlaces  several  inches  or 
even  several  feet.  The  bulk  of  the  wall  rock  is  ferruginous  chert  containing 
usually  less  than  30  per  cent  of  iron  oxide  and  60  to  70  per  cent  of  silica. 
Transition  into  the  ore  is  represented  almost  entirely  by  the  change  in  the 
relative  proportions  of  these  minerals.  The  discussion  of  the  origin  of  the 
ore  on  a  subsequent  page  will  include  an  account  of  the  genetic  relations  of 
the  cherts  to  the  iron  ores.  An  attempt  was  made  to  ascertain  whether  or 
not  any  particular  kind  of  ferruginous  chert  is  uniformly  represented  by  any 
particular  kind  of  iron  ore,  but  without  any  considerable  success.  In  the 
Biwabik  mine  a  hard  yellow  ferruginous  chert  (PL  XI,  fig.  A)  may  be 
observed  to  grade  into  the  yellow  ocher  of  the  deposit.  The  brown,  red, 
and  black  hematites  seem  to  grade  impartially  into  any  of  the  gray  or 
reddish  ferruginous  cherts.  The  ferruginous  cherts  in  the  wall  rock  of  the 
Mountain  Iron  mine  may  be  seen  to  grade  directly  into  purplish  slaty  ores 
with  white  specks.  Finally,  the  slate  in  the  wall  rock  may  be  observed  to 
grade  into  the  paint-rock  layers  within,  below,  and  above  the  ore  deposits. 
Evidence  of  the  correspondence  of  slate  and  paint  rock  has  been  noted  by 
several  of  the  mining  engineers  of  the  range,  although  some  of  them  object 
to  using  the  term  slate,  preferang  to  keep  this  term  for  the  overlying 


234  THE  MESABI  IRON-BEARING  DISTRICT. 

Virginia  slate.  The  equivalence  of  the  paint  rock  and  slate  is  indicated  by 
the  actual  transition  to  be  observed  at  a  number  of  cases,  and  also  by  their 
structural  relations.  The  sump  of  the  pump  shaft  of  the  Penobscot  mine  is 
bottomed  in  slate  under  298  feet  of  ferruginous  chert.  At  the  bottom,  of 
the  adjacent  ore  deposit  at  the  same  horizon  is  a  zone  of  paint  rock.  In  the 
Biwabik  mine  there  is  a  capping  of  paint  rock.  East  and  west  along  the 
strike  of  the  paint  rock  there  is  found  black  slate.  Also  a  well  driven  in 
the  paint  rock  and  ore  600  feet  south  of  the  southernmost  pit  pierces  typical 
black  slate  at  a  depth  which  ought  to  show  paint  rock  if  the  pauit  rock 
continues  southward  with  approximately  the  dip  it  shows  in  the  mine. 

At  the  bottom  of  the  deposit  the  ore  is  usually  in  sharp  contact  with 
feiTuginous  chert,  with  practically  no  gradation,  but  paint  rock  is  also  fre- 
quently found  immediately  underlying  the  ore.  This  may  be  in  single 
thin  seams  a  fraction  of  an  inch  or  several  inches  in  thickness,  or  may  be  in 
several  thin  layers  interleaved  with  the  ore  in  a  zone  several  feet  thick. 
Beneath  the  paint  rock,  and  to  a  certain  extent  mixed  in  with  it,  is  some 
variety  of  fen-uginous  chert  or  ferruginous  slate,  principally  the  former. 
Toward  the  sides  of  the  channels  the  paint  rock  between  the  ore  and  the 
ferruginous  chert  becomes  less  abundant  or  altogether  disappears,  and  the 
ore  rests  directly  upon  the  ferruginous  chert.  Attention  has  been  called  to 
the  fact  that  the  sides  of  the  trough  are  in  a  series  of  steps.  Oil  these  steps 
there  is  seldom  any  paint  rock  separating  the  ferruginous  chert  from 
the  ore. 

DRAINAGE. 

Because  of  the  bedded  and  jointed  structure  of  the  ore  deposits  water 
is  able  to  pass  through  them  freely.  Water  probably  flows  along  the  beds 
more  freely  than  across  them,  for  the  bedding  partings  present  more 
continuous  openings  than  the  joints,  which  in  the  ore  deposits  are  irregular 
and  discontinuous,  and  frequently  cut  off  by  soft  impervious  layers,  which 
have  yielded  to  deformation  by  bending  rather  than  by  jointing,  and, 
moreover,  certain  layers — for  instance,  limonite  layers — are  themselves  very 
porous.     Locally,  however,  the  flowage  along  joints  is  dominant. 

The  ore  deposits  are  at  present  in  part  above  water  level  and  iii  part 
l)clo\v.  On  the  upper  slopes  the  deposits  are  largely  above  water  level,  on 
tlic  lower  slopes  below  water  level,  although  there  are  many  exceptions  to 


THE  IRON-ORE  DEPOSITS  235 

both.  Thus  it  is  that  certain  mines  are  comparatively  dry  throughout  the 
year  while  others  are  permanently  wet.  A  good  instance  of  this  appears  at 
Virginia,  where  the  Columbia  mine  on  low  ground  receives  a  vast  quantity 
of  water  while  the  mines  on  high  gTOund  adjacent  are  comparatively  dry. 
The  pumping  of  vast  quantities  of  water  from  mines  below  the  level  of 
ground  water  has  materially  reduced  the  general  level  of  ground  water  in 
this  and  adjacent  areas.  The  Penobscot  mine,  for  instance,  discharges  in 
the  neighborhood  of  5,000  gallons  per  minute,  and  thereby  drains  the 
mines  to  the  north  and  west.  The  cessation  of  pumping  at  the  Penobscot 
mine  would  immediately  raise  the  level  of  ground  water  in  the  adjacent 
mines  were  pumping  not  begun  in  them.  At  the  Biwabik  mine  the 
level  has  been  lowered  from  75  feet  below  the  surface  to  150  feet  below 
the  surface.  Below  the  level  of  ground  water  the  amount  of  water  to 
be  handled  in  the  deposits  varies  with  the  depth  below  the  level  because 
of  the  increased  head  and  increased  contributing  area  in  the  shafts. 
Many  mines  show  increased  flow  toward  the  bottom.  Before  any  artificial 
openings  were  driven  into  the  ore  deposits  the  flowage  along  the  bottom 
may  not  have  been  any  greater  than,  if  indeed  so  great,  as  in  upper  portions, 
as  shown  by  fig.  11,  described  in  Chapter  IX.  The  outlet  for  the  water 
was  then  at  a  higher  level,  near  the  surface  of  the  formation. 

Above  the  level  of  ground  water  the  ore  contains  water,  but  is  not 
saturated.  In  this  zone  the  amount  of  water  in  the  ore  increases  from  the 
top  down.  In  the  Mountain  Iron  mine  the  amount  of  hygroscopic  moisture 
in  the  ore  is  said  by  A.  T.  Gordon,  chemist  in  the  mine,  to  vary  from  6  per 
cent  in  the  upper  part  of  the  deposit  to  15.  per  cent  in  the  lower  part. 
During  periods  of  great  precipitation  the  amount  of  water  in  the  deposits 
above  the  level  of  ground  water  is  increased  and  at  such  times  also  the 
level  of  ground  water  is  raised.  Where  mine  openings  have  been  made 
above  the  level  of  ground  water  the  amount  of  water  contributed  to  the 
openings  during  such  periods  is  of  course  increased. 

Many  deposits  in  the  range,  though  not  all  by  any  means,  lie  under 
surface  depressions  or  surface  drainage  channels,  due  to  development  in 
original  rock  synclines,  as  shown  on  a  subsequent  page,  or  to  gouging  out 
by  erosion  to  a  greater  extent  than  the  adjacent  harder  rocks.  Glacial 
drift  has  tended  to  obscure  these  rock  depressions,  and  does  so  completely 
in  many  places.     Where  open   cuts  have  been  made  in  deposits  under- 


236  THE  MESABI  IKON-BEARING  DISTRICT. 

Ij'ing  sucli  di-ainage  basins  great  precautious  have  to  be  takeu  to  guard 
agaius  flooding  from  this  surface  drainage.  In  1900  the  Mountain  Iron  open 
cut,  which  by  pumping  is  ordinarily  kept  above  the  level  of  ground  water, 
received  70,000,000  gallons  of  water  in  a  few  hours.  At  the  same  time  the 
Fayal  mine  fared  nearly  as  badly.  The  extensive  drainage  ditches  to  be 
seen  about  the  open  pits  testify  to  the  tendency  for  increased  flow  at  such 
times. 


CHAPTER  IX. 

ORIGIN  OF  THE  IRON  ORES. 

GElSnERAi  STATEMENT. 

The  iron  ores  have  come  chiefly  from  the  alteration  of  rocks  made  up 
of  minute  granules  of  green  hyclrated  ferrous  silicate,  which  we  have  called 
greenalite  (see  pp.  101-115  and  Pis.  VIII,  IX,  and  XIII).  A  small  part  of 
the  ore  has  resulted  from  the  alteration  of  siderite.  The  proof  of  the 
development  of  the  ores  from  greenalite  is,  briefly,  as  follows : 

1.  All  stages  of  the  alteration  from  the  fresh  greenalite  granules  to  the 
other  phases  of  the  iron  formation,  including  the  ores,  are  to  be  observed. 

2.  Much  of  the  ore  and  associated  rocks  show  traces  of  the  greenalite 
granules.  The  granules  may  be  represented  b}^  iron  oxide,  by  chert,  by 
actinolite  or  griinerite,  by  unaltered  greenalite,  by  any  combination  of  these 
substances,  or  by  any  gradation  phase  between  them. 

3.  Greenalite  is  one  of  the  less  stable  of  the  iron-formation  materials, 
and  its  alterations  to  the  other  phases  of  the  iron  formation  would  be  chem- 
ically characteristic  of  conditions  under  which  the  iron  formation  has 
existed  during  the  concentration  of  the  ore.  The  reverse  change  would 
not  be  probable  under  such  conditions.  Greenalite  is  found  in  parts  of  the 
iron  formation  which  have  suffered  the  least  alteration;  i.  e.,  in  association 
with  slate  layers  in  the  iron  formation  or  just  below  the  Virginia  slate. 

The  proof  of  the  development  of  a  small  portion  of  the  ore  from  sid- 
erite is,  briefly,  as  follows:  Associated  with  the  greenalite  and  slaty  layers 
in  the  least-altered  portions  of  the  iron  formation  are  thin  layers  of  iron 
carbonate,  some  of  which,  from  interlamination  with  the  easily  soluble 
greenalite,  probably  is  original  and  not  the  product  of  alteration  of  green- 
alite.. Near  Birch  Lake  undoubted  fine-banded  original  carbonate  is 
found.  If  iron  carbonate  is  now  found  in  the  unaltered  portions  of  the 
iron  formation,  it  must  have  originally  occurred  in  the  more  altered 
portions.     Its  alteration  to  the  other  phases   of  the  iron  formation  now 

237 


238  THE  MESABI  IRON-BEARING  DISTRICT. 

observed  would  be  chemically  characteristic  of  the  conditions  under  which 
the  iron  formation  has  existed.  Certain  of  the  finely  banded  ores — as,  for 
instance,  some  of  those  at  the  Mountain  Iron  mine — may  have  thus  resulted, 
although  there  is  no  evidence  in  the  ores  themselves,  aside  from  their  fine 
banding,  suggestive  of  development  from  banded  carbonate.  The  fact  that 
the  ores  of  the  Penokee-Grogebic  range  have  been  proved  mainly  to 
develop  from  u'on  carbonate  further  suggests  the  probability  of  such  a 
change  having  occurred. 

The  development  of  the  iron  ores  from  greenalite  has  consisted  in  the 
breaking  up  of  the  greenalite  into  its  constituents — mainly  protoxide  of  iron 
(FeO),  silica  (SiOj),  water  (H2O) — the  partial  or  complete  oxidation  and 
hydration  of  the  protoxide  of  iron,  and  the  abstraction  of  the  silica,  probably 
much  of  it  as  colloidal  silicic  acid  (H^SiO^),  in  solution.  Where  the  original 
rock  was  siderite  the  development  of  the  ores  occurred  in  a  similar  way. 
It  consisted  in  the  breaking  up  of  the  siderite  into  ferrous  iron  and  carbon 
dioxide,  the  partial  or  complete  oxidation  and  hydration  of  the  protoxide 
of  u-on,  and  the  abstraction  of  carbon  dioxide  as  carbonic  acid  in  solution. 

Where  the  oxidation  has  been  complete,  the  sesquioxides  have  devel- 
oped; these  are  usually  somewhat  hydrated.  Where  the  oxidation  has 
been  partial,  magnetite  has  developed. 

The  derivation  of  the  iron  ores  from  greenalite  and  siderite  has  been 
brought  about  through  the  agency  of  surface  weathering  and  of  waters 
percolating  thi-ough  the  formation  both  above  and  below  the  level  of 
ground  water.  This  is  shown  by  the  nature  of  the  chemical  changes  which 
the  ore  and  associated  rocks  of  the  iron  formation  give  evidence  of  having 
undergone,  by  the  fact  that  the  silica  and  iron  have  been  transported  and 
rean;anged  and  the  silica  largely  abstracted  from  what  are  now  the  ore 
bodies,  by  the  frequent  occurrence  of  the  ore  in  nodules  and  in  stalactitic 
forms  in  cavities,  by  the  occurrence  of  the  ore  in  underground  drainage 
channels  in  the  iron  formation,  and  finally  b}^  the  fact  that  water  may  now 
be  observed  coursing  through  the  formation  bearing  mineral  constituents 
indicative  of  such  changes. 

The  greenalite  rock  itself  is  a  sedimentary  oceanic  deposit,  as  shown  liy 
its  bedded  character  and  its  interstratification  and  conformity  with  ordinary 
slate  and  quartzite.  Its  development  is  believed  to  be  both  by  chemical 
and  oi'ganic  processes. 


ORIGIN  OF  THE  IRON  ORES.  239 

The  above  general  outline  of  the  genesis  of  the  iron-ore  deposits  is 
filled  in  below.  The  development  will  be  described  in  the  order  of  sequence 
of  events,  and  the  first  subject  to  be  considered  in  detail  is  naturally  the 
origin  of  the  greenalite  granules. 

ORIGIN    OF    THE     GBEEKALITE     GRANULES. 

GREENALITE  A   SEDIMENTARY  DEPOSIT. 

The  rocks  containing  greenalite  grade  into  quartzite  below,  into  slate 
above,  into  slate  laterally,  and  contain  interstratified  slate  layers.  They 
show  bedding  which  is  strictly  conformable  to.  that  of  the  associated  sedi- 
mentary rocks.  There  is  therefore  no  question  that  the  greenalite  of  the 
iron  formation  is  a  sedimentary  deposit.  The  occurrence  of  greenalite  or 
its  altered  equivalents  between  quartzite  and  slate  would  indicate  the  con- 
ditions of  its  deposition  to  be  somewhat  intermediate  between  those  favor- 
able to  sand  deposition  and  those  favorable  to  mud  deposition. 

SIMILARITY  OF  GREENALITE  TO   GLAUCONITE. 

Spurr,  in  his  work  on  the  Mesabi  range  in  1894,"  noted  the  similarity 
of  the  greenalite  granules  here  described  with  glauconite,  a  green  silicate  of 
iron  and  potassium,  found  in  formations  of  the  most  various  ages  and 
brought  up  in  dredging  operations  from  the  sea  bottom  near  the  edge  of 
the  continental  shelf,  where  it  occurs  in  all  stages  of  development  by  depo- 
sition in  the  interiors  of  foraminifera  and  other  organisms,  and  possibly  in 
small  part  developed  as  entooliths  and  concretions  independently  of  organ- 
isms. In  color,  size,  shape,  and  optical  properties  the  two  substances  are 
almost  identical,  but  in  other  respects  they  differ. 

Murray  and  Renard,  who  have  investigated  modern  glauconite  depos- 
its tlxrough  the  dredgings  of  the  Challenger  expedition,  emphasize  the  char- 
acteristic accompaniments  of  glauconite.  "Glauconite  is  almost  always 
accompanied  by  quartz,  orthoclase  (often  kaolinized),  white  mica,  plagio- 
clase,  hornblende,  magnetite,  garnet,  epidote,  tourmaline,  zircon,  and  frag- 
ments of  ancient  rocks,  such  a,s  gneiss,  mica-schists,  chlorite  rocks,  granite, 
diabase,  etc.  In  addition  to  these  minerals,  there  seems  always  to  be 
associated  with  glauconite,  in  modern  deposits,  a  considerable  quantity  of 
organic  matter,  often  apparently  of  a  vegetable  nature.     The   glauconitic 

a  Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  10. 


240 


THE  MESABI  IRON-BEARING  DISTRICT. 


grains  frequently  contain  traces  of  phosphate  of  lime  and  make  up  a  con- 
siderable part  of  some  phosphatic  nodules,  so  that  phosphate  of  lime  may 
be  said  to  be  one  of  its  constant  accompaniments.""  No  trace  of  these  sub- 
stances remains  in  the  greenalite  deposits  under  discussion.  In  thickness  of 
the  deposits  the  glauconite  and  greenalite  deposits  differ.  The  former  are 
nowhere  found  unmixed  with  foreign  material  with  a  thickness  exceeding 
35  feet,  while  the  gi'eenalite  granules  in  the  Mesabi  district  have  made  up 
a  deposit  with  a  thickness  of  1,000  feet  or  more,  with  only  thin  layers 
af  mud. 

In  the  fundamental  property  of  composition  they  are  dissimilar.  Spurr 
himself  noted  this  dissimilarity  in  composition,  but  in  view  of  the  wide 
variety  in  composition  of  glauconite,  and  the  similarity  in  ])hysical  prop- 
erties, concluded  that  the  material  was  glauconite.  A  number  of  anah^^ses  of 
so-called  glauconite  or  greensand  are  given  below.  Some  of  them  are  not 
reliable,  but  it  is  scarcely  possible  to  make  a  satisfactory  separation  of  the 
good  and  bad  without  details  as  to  methods. 

Analyses  of  glauconite. 


I 

II 

Ill 

IV 

v 

VI 

VII.... 
VIII... 

IX 

X 

XI 

XII.... 
XIII... 
XIV... 
XV.... 

XVI  ... 

XVII  .... 

XVIII  . . . 

XIX 

XX 


SiOo. 


58.17 
53.46 
49.10 
50.80 
48.99 
49.40 
50.20 
47.60 
49.50 
43.60 
46.91 
50.62 
54.18 
50.42 
49.09 
50.70 
46.58 
56.70 
48.45 
.51.50 


AljOa. 


10.09 

5.00 

7.05 

6.70 

6.40 

7.10 

1.50 

4.20 

3.20 

5.10 

7.04 

3.80 

7.15 

4.79 

15.21 

19.80 

11.45 

13.32 

6.30 

6.40 


Fe.Os. 


23.60 
21.80 
25.80 
20.07 
28.10 
21.60 
22.20 
32.80 
23.06 
21.03 


19.90 
10.56 


PeO. 


18.75 

21.  78 

3.25 

3.10 

4.80 

3.80 

4.20 

3.00 

6.80 

3.00 

2.64 

6.03 

20.16 

5.96 

3.06 

8.60 

20.61 

20.10 

24.31 

24.30 


MgO. 


3.37 
6.21 


4.20 
Spur. 


1.40 


1.50 
4.40 


4.08 
2.28 
2.65 
3.70 
1.27 
1.18 


Trace, 


CaO. 


0.78 


2.40 


2.95 


NaoO. 


0.91 


3.21  ;     .21 


.55 


2.49 
1.62 


1.21 
.50 
.98 


KoO. 


3.37 
8.79 
5.75 
3.10 
5.18 
5.75 
5.90 
4.60 
8.00 
5.60 
7.31 
7.14 
7.97 
7.87 
6.05 
8.20 
6.96 


HoO. 


12.01 
9.96 


6.25 
4.76 

10.10 
9.80 
8.98 

12.75 
8.60 

14.70 
9.50 
7.70 
4.71 
9.14 
5.74 
5.28 

11.64 
8.50 
9.66 


MnO.    MgCOa.  CaCO: 


Total. 


.!  0.57 


8.40 
7.70 


0.54 


100. 00 
100. 00 
98.85 
99.50 
100. 93 
98.87 
98.50 
99. 50 
99.  20 
99  30 
93 


99 

99.86 

99.28 

99.92 

100.  02 

100.  00 

100. 00 


99.47 
99.86 


"Report  of  the  voyage  of  H.  M.  S.  ChaUcngcr,  1873-1876,  Deep  Sea  Deposits,  p.  383. 


ORIGIN  OF  THE  IRON  ORES. 


241 


Analyses  of  glauconite — Continued. 


SiO.,. 

AI2O3. 

FeoOs. 

FeO. 

MgO. 

CaO. 

Na-O. 

K.O. 

H,0. 

MnO. 

MgCOs. 

CaCOa. 

Total. 

XXI 

50.75 
53.26 
57.56 
58.74 
49.42 
51.24 
49.76 
46.90 
52.86 
56.62 
50.85 
51.  80 
55. 17 
27.74 
48.95 
4:}.  75 
49.53 
51.00 
47.88 
42.32 
41.02 
42.35 
47.59 
52.96 

6.50 

3.85 

6.56 

4.71 

10.23 

12.22 

8.18 

4.06 

7.08 

12.54 

8.92 

8.67 

8.12 

13.02 

7.66 

7.82 

5.84 

9.93 

14.94 

16.51 

22.19 

21. 43 

17.99 

12.76 

16.01 
13.44 
16.00 
27.09 

7.20 
15.63 
24.40 
24.21 
21.59 
39.93 
23.43 
22.26 
20.06 
18.69 
17.13 
18.91 
18.49 

8.17 
13.95 
13.56 

22.14 
24.15 
20.13 
21.06 
3.00 
3.06 
3.77 
3.60 
19.48 
1.18 
1.66 
1.54 
1.95 
1.76 
1.32 
2.36 
5.95 
1.98 
2.68 
2.80 
2.06 
5.78 
3.70 
2.34 

12.96 
5.36 
4.88 
3.26 
7.91 
7.50 
7.57 
6.16 
2.23 
2.52 
4.21 
3.86 
3.36 
.95 
9.54 
9.01 
9.31 
7.66 
8.04 
7.49 
5.74 
7.16 
7.21 
8.69 

7.50 
10.12 
8.17 
9.79 
8.08 
8.20 
9.82 
9.25 
8.43 
6.84 
5,55 
5.68 
5.76 
10.  85 
4.93 
5.16 
4.91 
5.83 
5.91 
7.48 
7.88 
4.45 
5.27 
4.91 

99  85 

XXII  . . . . 

1.10 
1.70 
1.48 
3.78 
3.93 
3.97 

.70 
2.90 
2.49 
3.13 
3.04 
2.83 
4.62 
2.97 
3.25 
2.92 
3.85 
2.45 
1.74 

.69 
1.93 
1.80 
4.11 

1.73 

1.04 

.92 

.31 

.10 

.41 

.20 

Trace. 

1.69 

1.26 

1.27 

1.34 

1.19 

.57 

.56 
.87 
.56 
2.20 
1.96 
8.37 
1.22 

1.60 

.26 
.31 
.52 
1.28 
Trace. 
.90 
.25 
.25 
.27 
.62 
.98 
.30 
.46 
.35 
.43 
.42 
.38 
.43 
.42 
.47 

101  17 

XXIII . .  . 

100.  04 

XXIV  ... 

XXV  .... 

99.96 
99.00 

XXVI  ... 

100. 00 

XXVII  .. 

100. 00 

XXVIII  . 

99.24 

XXIX  ... 

100.  IS 

XXX.... 

Trace. 

100.  41 

XXXI... 

Trace. 

100. 23 

XXXII  . . 

Trace. 

100  32 

XXXIII  . 

Trace. 

100  39 

XXXIV  . 

Trace. 

100. 68 

XXXV  .. 

XXXVI  . 

100. 35 
99.91 

XXXVII. 

99. 54 

XXXVIII 

100. 16 

XXXIX  . 

100  02 

XL 

99  87 

XLI 

100  41 

XLII  .... 

100  07 

XLIII  ... 

99  15 

XLIV.... 

100. 10 

I.  Greensand,  from  Biiderich,  between  Unna  and  Werl.     Dechen,  Xat.-hist.  Ver.  Bonn,  1855, 
Vol.  XII,  p.  176. 
II.  Greensand,  from  Dortmund  in  the  direction  of  Witten.     Marck,  ibid.,  p.  266. 

III.  Bindlacher  Mountain,  near  Bayreuth.    Haushofer,  Journ.  pr.  Chem.,  1866,  Vol.  XCVII,  p.  353. 

IV.  Sorg,  near  Kronach  in  Oberfranken.     Ibid. 
V.  Ortenburg,  near  Passau.     Ibid. 

VI.  Roding  at  Cham,  in  the  Oberpfalz.'    Ibid. 
VII.  Ebendaher.     Ibid. 
VIII.  Benedictbeuern.     Ibid. 
IX-X.  Kressenberg.     Ibid. 

XI.  Insel  Gozzo.     Bamberger,  Tscherm.  Mitth.,  1887,  p.  271. 
XII.   Havre.     Haushofer,  Journ.  pr.  Chem.,  1867,  Vol.  CII,  p.  36. 
XIII.  Villers  sur  Mer,  Dep.  Calvados.     Pisani,  Cloizeaux  Min.,  1862,  p.  .542. 
XIV.  Antwerp.     Dewalque,  Soc.  geol.  Belgique,  1874,  Vol.  II,  p.  3. 
XV.  Ashgrove,  near  Elgin,  in  Scotland.  F.  Heddle,  Trans.  Roy.  Soc.  Edinb.,  1879,  Vol.  XXIX,  p.  79. 
XVI.  Island  of  Orleans,  Quebec.     Hunt,  Geol.  of  Can.,  1863,  p.  487. 
XVII.  Red  Bird,  Mo.     Ibid. 

XVIII.  (iay  Head,  Massachusetts.     S.  L.  Dana,  Hitchcock's  G.  R.  Mass.,  1841,  p.  93. 
XIX.  Cauley's  pits,  at  Woodstown,  X.  J.     Rogers,  Geol.  Rept.  N.  J.,  p.  201-204. 
XX.  ScuUtown,  N.  J.     Ibid. 

"  MON    XLIII — 03 16 


242  THE  MESABI  IRON-BEARING  DISTRICT. 

XXI.  Polk  Hill,  Burlington  County,  N.  Y.     Ibid. 

XXII.  New  Jersey,  southeast  of  Philadelphia.     Fisher,  Am.  Journ.  Sci.,  1850,  Vol.  IX,  p.  83. 

XXIII.  Coal  Bluffj  Alabama.     Mallet,  Am.  Journ.  Sci.,  1857,  Vol.  XXIII,  p.  181. 

XXIV.  Gainesville,  Ala.     Ibid. 

XXV.  Svir  R,  Russia.     A.  Kupffer,  J.  B.  Ch.,  1307,  1870. 
XXVI.  Ontika,  Russia.     Ibid. 
XXVII.  Grodno  Valley,  Russia.     Ibid. 
XXVIII.  Agulhas  Bank.     Giimbel,  Ber.  Ak.  Miinchen,  1886. 
XXIX.  French  Creek,  Pennsylvania.     Knerr  and  Schoenfeld,  Am.  Ch.  J.,  Vol.  VI,  1884. 
p.  412. 
XXX-XXXIII.  Challenger  dredging,  lat.  34°  13'  S.,  long.  151°  38'  E.,  410  fathoms. 

XXXIV.  Challenger  dredging,  lat.  11°  38'  15"  S.,  long.  143°  59'  38"  E.,  155  fathoms. 
XXXV.  Padi,  Gouv.  Saratow.     K.  Glinka,  Zeitschr.  fiir  Kryst.,  Vol.  XXX,  1899,  p.  391. 
XXXVI.  Xasonovo,  Giouv.  Smolensk.     Ibid. 
XXXVII.  Ural.     Ibid. 

XXXVIII.  Traktemiroff,  Gouv.  Kiew.     Ibid. 
XXXIX.  Tschernofskoje,  Gouv.  Nischni-Novgorod.     Ibid. 
XL.  Karowo,  Gouv.  Kaluga.     Ibid. 
XLI.  The  same,  another  portion.     Ibid. 
XLII.  Kosolapowo,  Gouv.  Nischni-Novgorod.     Ibid. 
XLIII.  The  same,  another  portion.     Ibid. 
XLIV.  Udriass  in  Estland.     Ibid. 

It  is  apparent  from  the  above  analyses  that  so-called  "glanconite"  has 
a  very  indefinite  and  vai'iable  composition.  In  mineralog-ic  text-books  and 
in  the  work  of  Mnrray  and  Renard  on  modern  glauconite  deposits  the 
substance  is  described  as  ^'■probably  a  mixture."  Indeed,  the  variation  in 
composition  is  so  marked  as  to  lead  one  to  suspect  that  substances  of  dif- 
ferent origin  have  been  included  under  this  term.  The  main  variation  is 
in  the  iron.  In  most  of  the  samples  the  iron  is  largely  in  the  ferric  form, 
and  in  the  analyses  of  typical  glauconite  grains  collected  by  the  Challenger 
expedition  this  is  true  in  each  case.  Indeed,  Sir  John  Murray,  after  sum- 
marizing the  results  of  their  work,  makes  the  statement  that  "the  glauconite 
now  foi'ming  on  the  bottom  of  the  sea  is,  like  the  glauconite  of  geological 
formations,  a  hydrous  silicate  of  potash  and  of  ferric  oxide,  containing  always 
variable  quantities  of  alumina,  ferrous  oxide,  magnesia,  and  often  lime." 
Yet  in  analyses  of  so-called  glauconites  from  various  geologic  formations 
the  ferrous  iron  is  greatly  in  excess  of  the  ferric  iron. 

Comparing  the  analyses  of  glauconite  above  listed  with  those  of  the 
green  granules  of  the  Mesabi  iron  formation,  the  following  differences 
appear: 

The  amount  of  alumina  in  glauconite  averages  9  per  cent  in  the  above 
■  talile,  wliile  less  than  1  per  cent  is  found  in  the  greenalite  rocks,  and  tliis 
doubtfully  Ijelongs  with  the  greenalite. 


ORIGIN  OF  THE  IRON  ORES.  243 

The  ferric  iron  in  all  of  the  best  analyses  of  glanconite  is  in  greater 
percentage  than  the  ferrous  iron.  In  the  green  granules  from  the  Mesabi 
district  feme  iron  is  nearly  if  not  quite  absent.  The  percentage  of  metallic 
iron  in  glauconite  is  on  an  average  lower  than  that  of  the  green  granules 
of  the  Mesabi  district. 

Glauconite  contains  a  small  percentage  of  soda.  In  the  Mesabi 
granules  soda  is  entirely  lacking. 

Glauconite  contains  from  3  to  13  per  cent  of  potassa,  and,  indeed,  in 
descriptions  of  glauconite  in  standard  text-books  the  content  of  potassa  is 
noted"  as  a  characteristic  of  it.  In  the  green  granules  from  the  Mesabi 
district  potassa  is  entirely  lacking,  and  the  granules  are  so  fresh  and 
unaltered  as  to  preclude  the  idea  that  this  substance  was  originally  present 
and  has  been  removed. 

Prof.  F.  W.  Clarke,  chief  chemist  of  the  United  States  Geological 
Survey,  under  whose  direction  the  chemical  work  on  the  Mesabi  green 
granules  was  done,  kindly  makes  for  publication  here  the  following  state- 
ment concerning  the  chemical  differences  between  glauconite  and  greenalite 

THE   COMPOSITION   OF   GLAUCONITE   AND   GREENALITE. 
B}'  F.   W.  Clakke,  Chief  Chemist. 

"A  scrutiny  of  the  table  of  analyses  of  glauconite  compiled  by  Mr.  Leith 
reveals  at  tirst  sight  a  hopeless  discordance  of  data.  This  is  due  to  the  fact 
that  glauconite,  as  actually  observed  and  analyzed,  is  never  piire  and  defi- 
nite, but  contains  a  variety  of  imdetermined  substances  commingled  with 
the  true  glauconite  silicate.  Some  of  the  analyses,  moreover,  are  old  and 
obsolete,  and  evidently  made  without  the  precautions  which  are  now  recog- 
nized to  be  essential.  Iron,  for  instance,  is  often  all  recorded  as  ferrous,  no 
determination  of  the  fact  having  been  made.  The  figures  for  "water"  often 
represent  only  loss  on  ignition,  a  method  of  estimation  of  notoriously  falla- 
cious chai-acter.  In  order  to  discuss  the  composition  of  glauconite  the 
analyses  must  be  carefully  sifted  and  criticised,  for  much  of  the  pubhshed 
material  is  entirely  worthless. 

"  The  recent  series  of  glauconite  analyses  by  Glinka,"  made  upon  sam- 
ples from  various  Russian  localities,  is  of  great  value,  for  the  reason  that 
the  glauconite  grains  were  carefully  separated"  from  admixed  impurities  by 

aZeitschr.  Kryst.  u.  Min.;  Vol.  XXX,  p.  390. 


244 


THE  MESABI  IRON-BEARING  DISTRICT. 


means  of  Thoulet's  solution,  and  the  fig'ures  indicate  that  the  work  was 
done  with  care.  The  analyses  are  ten  in  number,  and  the  five  best,  XXXV- 
XXXVIII,  XLIV,  in  the  table  on  page  241,  made  upon  material  dried  at 
100°,  are  as  follows: 

Analyses  of  glauconite. 


Constituent. 

XXXV. 

XXXVI. 

XXXVII. 

XXXVIII. 

XLIV. 

SiOa 

48. 95 

7.66 

23.43 

1.32 

.57 

2.97 

9.54 

.98 

4.93 

49.75 
7.82 

22.26 
2.36 

49.53 

5.84 

20.06 

5.95 

.56 

2.92 

9.31 

.46 

4.91 

51.00 
9.93  '• 
18.69 
1.98 

.87    . 
3.85 
7.66 
.35 

5.83 

1 

52.96 

AljO,                                       

12.76 

Fe,0,                                   

13.  56 

FeO 

2.34 

CaO 

MeO 



3.25 

9.01 

.30 

5.16 

4.11 

KjO 

8.69 

Na,0 

.47 

HjO 

4.91 

Total 

100.  35 

99.91 

99.54 

100. 16 

99.80 

"  From  these  figures  the  following-  molecular  ratios  can  be  derived: 


Constituent. 

XXXV. 

XXXVI. 

XXXVII. 

XXXVIII. 

XLIV. 

SiOii 

0.816 
.216 
.103 
.117 
.276 

0.846 
.216 

.114 
.101 
.287 

0.825 
.182 
.165 
.106 
.273 

0.850 
.214 
.138 
.087 
.324 

0.883 

R..0, 

.210 

RO 

.135 

E„0 

.099 

HjO 

.273 

"Since  RO  and  RoO  vary  reciprocally,  replacing  each  other,  these 
factors  may  all  be  reduced  to  the  general  equivalent  of  R',  giving  tlie 
subjoined  empirical  formulae: 

XXXV R'«oR'%2Si8,„0,5„„.  276  aq. 

XXXVI R'«oR'"«.SwO,555.  287  aq. 

XXXVII R'.,.R'"3o,Si„,50„„T.  273  aq. 

XXXVIII R',5oR'%8Si„5„0,5„;.  324  aq. 

XXXIX RVR'"«„Si8sA™.  273  aq. 

"If  we  neglect  the  water  as  extraneous — that  is,  as  '  zeolitic '  or 
crystalline — and  therefore  not  a  part  of  the  silicate  molecule,  these  figures 
give  quite  closel}-  the  general  formula 

R'R"'Si.A)„,+aq., 

in  which  1!'  is  mainly  K,  K'"  is  mainl)'  Fe,  and  witli  the  usual  replacements 


ORIGIN  OF  THE  IRON  ORES. 


245 


of  these  radicals  by  others,  as  shown  in  the  analyses.  That  is,  glauconite, 
in  its  purest  forms,  must  be  regarded  as  a  metasilicate,  approximating  more 
or  less  closely  to  the  typical  compound 

KFe'"Si206+aq., 

which,  however,  like  many  other  silicate  molecules,  has  not  yet  been  found 
in  a  state  of  purity.  Such  a  compound  would  easily  lose  alkali  and  take 
up  water,  jnelding,  as  Glinka  observes,  a  ferruginous  clay  as  its  final 
product  of  alteration.  Many  of  the  observed  variations  in  the  composition 
of  glauconite  are  due  to  partial  alteration  of  this  very  obvious  kind.  The 
other  variations  represent  the  replacement  of  the  iron  salt  by  its  aluminic 
equivalent  and  of  the  potassium  salt  by  corresponding  compounds  of 
sodium,  magnesium,  and  ferrous  iron. 

"In  the  case  of  the  mineral  described  by  Mr.  Leith,  to  which  he  has 
given  the  name  of  '  greenalite,'  the  evidence  is  less  complete.  The  sub- 
stance is  so  intimately  commingled  with  chert  that  it  can  not  be  isolated 
by  ordinaiy  physical  means,  and  its  composition,  therefore,  is  only  to  be 
determined  from  that  of  the  soluble  portion  of  the  mixture.  This  portion, 
according  to  the  three  latest  analyses  made  by  Mr.  Steiger,  representing  the 
most  carefully  chosen  material,  has  the  composition  given  below."  The 
summation  gives  the  total  amount  of  mineral  decomposed  by  hydrocldoric 
acid  in  100  per  cent  of  the  rock.     Hygroscopic  water  is  rejected. 


Constituent. 

45758. 

45765. 

45766. 

SiO,                       

13.45 

.37 

15.00 

10.28 

2.33 

.28 

4.17 

2.04 

19.30 

.61 

13.83 

17. 57 

3.22 
None. 

5.74 

33.11 

AI2O3 

.56 

Fe,0< 

6.44 

FeO. 

30.93 

McO                                        

5.35 

CaO                                  

None. 

H2O                       - 

6.13 

CO,                                              -- 

Total             

47.92 

60.27 

82.52 

"Professor  Clarke's  discussion  was  written  without  an  opportunity  to  examine  slides  of  the  rocks 
analj'zed,  and  hence  no  account  is  taken  of  the  rock  alterations.  However,  the  alterations  are  slight 
and  in  no  way  invalidate  Professor  Clarke's  main  conclusion  that  the  substance  of  the  Mesabi  green 
granules  is  different  from  glauconite.  Indeed,  were  the  alterations  taken  into  account,  and  especially 
the  alteration  to  ferric  oxide,  the  basis  for  his  conclusion  would  be  strengthened.  On  pp.  108-115  the 
writer  has  discussed  the  analyses  with  reference  to  the  alterations. — C.  K.  L. 


246 


THE  MESABI  IRON-BEARING  DISTRICT. 


"To  compare  these  data  they  must  l3e  reduced  to  corresponding  terms. 
According-ly,  akimina  lias  been  recalculated  into  its  equivalent  of  ferric 
iron,  maffuesia  and  lime  into  ferrous  iron,  and  in  the  first  example  the 
carbon  dioxide  has  been  deducted,  with  its  corresponding  amount  of 
monoxide  bases.  Then,  recalculating  to  100  per  cent,  we  have  the  following 
figures  to  represent  the  composition  of  the  soluble  mineral : 


45758. 

45765. 

45766. 

Constituent. 

Composi- 
tion. 

Ratio. 

Composi- 
tion. 

Ratio. 

Composi- 
tion. 

Ratio. 

SiO : ,.. 

30.08 
34.  8.5 

25.  72 
9.35 

0.501 
.218 
.3.57 
.519 

30.49 

23.52 

36.92 

9.07 

0.508 
.147 
.513 
.503 

38.00 
8.40 

46.56 
7.04 

0.633 

FejOj  

.052 

FeO 

H^O 

.648 
.391 

Total 

100.  00 

100. 00 

100.  00 

"Although  these  ratios  are  not  concordant,  they  still  show  something 
radically  different  from  glauconite.  If  we  restate  them  in  the  shape  of 
empirical  formulae,  we  have — 

No.  4.5758 Fe"35,Fe'%6Si5oAoi3-     519  H,0 

No.  4.5765 Fe"5,3Fe'",s,Si5„80ic„o.     503  H^O 

No.  4.5766 Fe"e«Fe'"5..  Si63sO,„™.     391  H,0 

"The  first  two  of  these  expressions  give  quite  sharply  the  orthosilicate 
ratio.  The  third  represents  a  lower  stage  of  oxidation,  but  something 
which  is  still  far  too  high  for  a  metasilicate.  In  the  second  case  the  com- 
position approximates  to  the  simple  formula, 

Fe"'oFe"3(SiO,)3.     3  H^O, 

in  which  the  ratios  are  those  of  a  hydrated  ferroso-ferric  garnet.  In  the 
last  of  the  three  analyses  the  ferric  oxide  is  very  low  and  the  ferrous  oxide 
very  high,  which  suggests  the  possibility  that  the  original  mineral  may 
have  been  wholly  ferrous,  and  that  it  has  undergone  partial  oxidation  in 
the  other  samples.  The  ratios  FeOiSiOg  is  here  1:1,  indicating  a  possible 
hydrated  FeSiOg  as  the  normal  substance. 

"The  composition  of  greenalite,  then,  is  uncertain.  It  may  be  a  ferrous 
metasilicate,  or  it  may  be  a  ferroso-ferric  orthosilicate.  In  either  case  tin- 
miiicrnl  difters  fundnmontnllv  from  ghuiconito,  a  ])otnssium  ferric  nietasili- 


ORIGIN  OF  THE  IRON  ORES.  247 

cate,  and  can  not  be  united  with  the  latter  species.  Similarity,  or  even 
identity,  of  appearance  under  the  microscope  can  not,  in  substances  of  this 
kind,  offset  the  evidence  of  the  ratios." 

F.  W.  Clakke. 

Spun-'s  argument  that  the  green  granules  of  the  Mesabi  district  are 
glauconite  is  based  mainly  on  their  similarity  to  glauconite  in  color  and  in 
shape,  and  on  the  fact  that  substances  of  a  great  variet}^  of  composition  have 
been  called  glauconite  by  mineralogists,  making  it  allowable  for  him  to  use 
the  term  for  a  somewhat  exceptional  phase  in  the  Mesabi  district.  It 
is  shown  above  that  the  green  graniiles  of  the  Mesabi  differ  frOm 
glauconite  deposits  in  their  association,  in  their  thickness,  and  in  several 
features  of  their  composition.  It  is  further  shown  that  while  no  decisive 
proof  of  a  definite  chemical  composition  of  the  substance  of  the  green 
granules  in  the  Mesabi  district  has  been  brought  forward,  the  facts  are 
such  as  to  indicate  a  distinct  possibilit)^  that  the  green  granules  of  the 
Mesabi  have  a  definite  and  uniform  chemical  composition.  While  all 
these  differences  are  thought  to  be  more  or  less  significant,  the  most  sig- 
nificant difference  between  the  green  granules  of  the  Mesabi  district  and 
glauconite  granules  is  taken  to  be  the  entire  absence  of  potash  in  the  former. 
There  seems  to  be  the  fullest  authority  for  the  statement  that  potash  is  an 
essential  constituent  of  glauconite.  If  potash  is  an  essential  constituent 
of  glauconite,  then  its  absence  is  sufficient  warrant  for  the  conclusion 
that  the  substance  of  the  green  granules  of  the  Mesabi  district,  whatever 
its  origin,  is  not  glauconite.  As  the  substance  corresponds  to  no  other 
known  mineral  species  and  since  a  name  is  necessary  to  facilitate  discussion 
of  the  substance,  the  name  greenalite  has  been  coined,  as  noted  on  a  preced- 
ing page.  If  mineralogists  and  chemists,  in  view  of  the  differences  between 
the  two  substances  above  described,  still  think  it  desirable  to  stretch  the 
term  "glauconite"  to  cover  the  substance  under  discussion,  it  is  suggested 
that  the  name  "greenalite"  may  be  retained  as  a  varietal  name  under 
"glauconite." 

EXPLANATION   OF  THE  OCCURRENCE  OF  GREENALITE  IN   GRANULES. 

The  green  granules  of  the  iron-bearing  rocks  have  been  shown  not  to 
be  glauconite,  but  to  be  a  substance  with  a  different  composition  which  has 
been  called  greenalite;  but  the   occurrence  of  greenalite  in  granules  has 


248  THE  MESABI  IRON-BEARING  DISTRICT. 

not  been  explained.  Are  the  granules  normal  coucretious  with  radial  and 
concentric  structures  about  foreign  nuclei,  or  are  they  concretionary  or 
amorphous  growths  about,  within,  or  replacing  minute  organisms,  and  thus 
directly  analogous  to  glauconite? 

1.  None  of  the  fresh  green  greenalite  granules  show  any  traces  what- 
ever of  radial  or  concentric  structure  or  of  any  foreign  nucleus  which  is 
characteristic  of  concretionary  or  oolitic  structures.  Where  altered  in  a 
few  cases  the  iron  oxide  and  chert  or  the  different  kinds  of  iron  oxide  have 
a  rough  concentric  arrangement  which  may  indicate  that  the  original 
material  possessed  a  concretionary  structure,  but  which  more  jDrobably  has 
been  developed  secondarily  and  quite  independent  of  the  original  structure 
of  the  granule.  A  few  undoubted  concretions  consisting  of  concentric  layers 
of  iron  oxide  and  chert  about  a  nucleus  of  quartz  are  to  be  observed 
(PI.  XIII,  fig.  D).  A  concretion  of  this  kind  may  be  distinguished  at  a 
glance  from  the  greenalite  granules  or  their  derivatives,  even  when  the 
latter  have  rough  concentric  arrangement,  as  may  be  seen  by  comparison 
of  the  figures  of  PI.  XIII.  These  concretions  are  identical  in  shape  with 
those  described  and  figured  by  Van  Hise  as  characteristic  of  the  Peuokee- 
Grogebic  iron-bearing  formation,  in  which  they  have  developed  from  the 
alteration  of  iron  carbonate."  (See  PL  XVI,  fig.  A)  There  is  carbonate 
associated  with  greenalite  in  the  Mesabi  district,  and  it  is  not  impossible 
that  the  Mesabi  concretions  may  have  developed  from  carbonate,  although 
no  direct  evidence  of  this  has  been  observed.  ,The  true  concretions  of 
the  Mesabi  district  also  find  their  counterpart  in  concretions  in  the  widely 
distributed  Clinton  iron  ores,  in  which  they  probabl}^  developed  at  the 
time  of  the  deposition  of  the  ore.  They  are  similar  in  form  also,  though 
not  in  substance,  to  the  bavalite,  chamosite,  and  berthierine  of  European 
ores.''  Whatever  the  origin  of  the  concretions  in  the  Mesabi  district,  it  is 
clear  that  they  are  quite  independent  of  and  subordinate  to  the  greenalite 
granules,  lacking  evidence  of  radial  or  concentric  structures  or  nuclei 
characteristic  of  true  concretions. 

2.  The  similarity  of  the  granules  of  the  Mesabi  iron  formation  in 
general  aspect  to  glauconite  grains  and  to  certain  organic  granules  of  the 
Clinton  ores  brings  to  mind  very  strongly  the  possibility  that  the  shapes  of 
the  Mesabi  STanules    mav  be  determined    bv  similar  conditions.     While 


«Mon.  U.  S.  Geol.  Survey  Vol.  XIX,  1892.  ''See  Lacroix,  Minerals  of  Fiaiue,  |>.  401. 


PLATE  XXI. 


249 


PLATE    XXI. 

PHOTOMICKOGRAPHS    OF    GRANULES   AND    CONCKETIONAKT   STRUCTURES    IN    CLINTON 

IRON    OpES. 

Fig.  vl.— Granules  in  Clinton  iron  ore.  From  lower  bed  Sand  Mountain,  New  England  City,  Ga. 
Loaned  tay  C.  H.  Smyth,  jr.  Without  analyzer,  x  40.  Granules  of  black  and  dark-brown  hydrated 
hematite  stand  in  a  matrix  of  calcite.  The  latter  areas  within  the  granules  are  also  calcite.  Traces  of 
organic  shells  in  these  slides  are  abundant.  The  granule  a  little  to  the  right  of  the  center  shows  this 
especially  well.  There  can  be  no  doubt  as  to  the  fact  that  the  granules  are  for  the  most  part  replace- 
ments and  accretions  about  shells  and  particles  of  shells.  It  is  apparent  also  that  there  is  a  marked 
tendency  for  the  granules  to  take  on  rounded  and  aval  forms  regardless  of  the  shape  of  the  original 
particles  of  shell.  Note  the  remarkable  similarity  of  these  granules  in  shape  to  the  greenalite  granules 
illustrated  in  PL  XIII. 

Fig.  5.— Green  oolites  in  Clinton  ore.  From  Clinton,  N.  Y.  Loaned  by  C.  H.  Smyth,  jr. 
With  analyzer,  x  40.  Concentric  layers  of  chloritic  and  siliceous  substance,  of  various  shades  of  green 
and  yellow,  surround  angular,  subangular,  and  rounded  grains  of  quartz.  The  concentric  greenish 
and  yellowish  bands  under  crossed  nicols  show  black  crosses  characteristic  of  concretionary  structures. 
The  matrix  is  mainly  calcite,  but  there  are  present  also  small  particles  of  quartz. 

250 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XLIII   PL.  XXI 


PHOTOMICROGRAPHS   OF  GRANULES   AND  CONCRETIONARY   STRUCTURES   IN 

CLINTON   IRON   ORES. 


THE   MERIDEN   GHAVURE   CO. 


ORIGIN  OF  THE  IRON  ORES.  251 

the  greenalite  is  different  in  composition  from  glauconite,  it  might  still  be 
developed  in  much  the  same  way.  The  close  resemblance  to  g-lauconite 
grains  in  shape  and  phvsical  properties  other  than  specific  gravity  has 
alreadj-  been  noted.  Noting  the  remarkable  similarity  in  external  shape 
between  the  Mesabi  granules  and  granules  of  the  Clinton  ores  of  Wisconsin, 
the  writer  asked  Dr.  C  H.  Smyth,  jr.,  of  Hamilton  College,  Clinton,  N.  Y., 
for  slides  of  the  Clinton  ores,  which  occurr  so  widely  in  the  eastern  portion 
of  the  United  States,  and  which  he  had  described." 

Professcn-  Sm^i:h  kindly  furnished  the  slides  requested,  and  thus  enabled 
the  following'  comparison  to  be  made:*  In  the  Clinton  ores  two  kinds  of  gran- 
iiles  are  numero  is:  (a)  Normal  concretions  of  silica  and  iron  oxide  or  of 
silica  and  some  greenish  siibstance  with  a  ferrous  iron  base,  the  further 
composition  of  which  is  unknown,  about  a  nucleus  of  quartz.  (Fig.  B, 
PI.  XXI.)  These  are  analagous  to  the  few  true  concretions  observed  in 
the  Mesabi  district  and  to  the  concretions  of  the  Penokee-Gogebic  district, 
(b)  Accretions  of  iron  oxide  about  calcium  carbonate  shells  and  partial  or 
complete  replacements  of  the  shells,  in  either  case  without  or  nearly- 
without  radial  or  concentric  structures.  (Fig.  A,  PI.  XXI.)  The  size  is 
somewhat  greater  than  that  of  the  Mesabi  granules.  It  is  noticeable  that 
while  traces  of  shells  are  abundant  in  the  Clinton  granules  the  shapes  of  the 
granules  are  not  closely  dependent  upon  the  shape  of  the  shell  or  fragment. 
On  the  contrary  there  seems  to  be  a  uniform  tendency  for  the  granules  to 
develop  with  rounded  and  oval  outlines  no  matter  what  the  form  of  the 
shells  which  they  replace.  The  shapes  are  almost  identical  with  those  of  the 
normal  greenalite  granules  of  the  Mesabi,  as  a  comparison  of  Pis.  IX,  XIII, 
XV,  and  XXI  will  show.  The  similarity  in  shape  is  as  close  as  between 
greenalite  and  glauconite.  ■  The  crescent  shapes,  the  gourd  shapes,  the 
much  elongated  ovals,  and  rods,  which  are  seen  associated  with  the  round 
and  oval  forms  in  the  Mesabi  rocks  are  all  to  be  seen  in  the  Clinton  ores. 
The  calciiTui  carbonate  of  the  shells  in  the  interior  of  some  of  the  granules 
of  some  of  the  Clinton  ores  also  has  a  mottled  appearance  very  similar  to 
that  observed  in  some  of  the  Mesabi  slides  figured  in  PI.  XIV,  although 

«  Am.  Jour.  Sci.,  3d  series,  Vol.  XLIII,  1892,  pp.  487^96. 

6  In  this  connection  a  remarkable  coincidence  may  be  noted.  In  a  letter  of  the  same  date 
Professor  Smyth  asked  for  information  concerning  the  ilesabi  granules.  He  had  seen  a  brief 
preliminary  description  of  them,  by  the  writer,  in  Science,  and  had  lieen  struck  with  the  similarity  of 
certain  of  their  features  with  those  of  the  Clinton  ores,  with  which  he  was  familiar. 


252  THE  MESABI  IRON-BEARING  DISTRICT. 

beyond  this  there  is  no  evidence  that  the  two  are  the  same..  In  both  the 
Clinton  ores  and  the  Mesabi  rocks  a  not  ■uncommon  feature  is  the  accretion 
of  a  considerable  number  of  granules  into  somewhat  irregular  pebble-like 
ao-2-res-ates  which  have  been  waterworn  as  a  whole  and  deposited  parallel 
to  the  bedding.  The  Chnton  granules  differ  from  the  unaltered  Mesabi 
granules  in  that  they  are  either  iron  oxide  entirely  or  partly  iron  oxide  and 
partly  calcite,  while  the'  Mesabi  granules  consist  of  ferrous  silicate  when 
fresh  and  largely  of  iron  oxide  and  chert  when  altered.  The  Clinton  ores, 
in  their  present  form,  may  not  be  concretions  or  replacements  subsequent 
to  their  deposition,  for  they  have  uniform  composition  in  thin  beds  over 
great  areas,  which  could  not  be  the  case  were  they  subsequently  concen- 
trated through  underground  water  or  other  agencies.  They  may  well  be 
compared  with  the  fresh  greenalite  granules  of  the  Mesabi  which  also  have 
undergone  no  concentration,  rather  than  with  the  altered  granules.  If 
dm-ing  the  deposition  of  the  Clinton  ores  the  numerous  minute  shells 
had  been  surrounded  and  replaced  by  iron  sihcate  instead  of  iron  oxide, 
greenalite  granules  identical  with  those  in  the  Mesabi  district  may  have 
resulted. 

It  is  concluded  that  the  greenalite  granules  of  the  Mesabi  district, 
while  associated  with  a  few  typical  concretions,  are  not  for  the  most  pait 
normal  concretions  with  radial  or  concentric  structures  about  an  inorganic 
nucleus;  that  from  their  remarkable  similarity  in  shape  to  glauconite 
grains  which  are  mainly  developed  as  replacements,  secretions,  or  accretions 
about  minute  organisms  (although  perhaps  partly  in  other  ways),  and  their 
similarity  to  the  accretions  about  and  replacements  of  shells  in  the  Clinton 
ores,  they  may  owe  their  shape  mainly  to  similar  development  either  witliin 
or  about  or  replacing  minute  organisms  of  the  variety  commensurate  with 
that  now  observed  both  in  modern  glauconite  deposits  and  in  the  Clinton 
ores;  that  the  development  of  greenahte  instead  of  glauconite  or  iron  oxide 
was  largely  a  matter  of  substances  present  which  were  available  for 
accretion,  secretion,  or  replacement.  However,  the  absolute  absence  of 
organic  structures,  aside  from  the  suggestive  similarity  in  shape  to  granules 
of  known  organic  origin,  must  still  be  kept  in  mind,  and  the  conclusion 
here  given  must  be  regarded  as  a  tentative  one,  lacking  suthcient  basis  of 
dii-('ct  observation  to  render  it  final. 


ORIGIN  OF  THE  IRON  ORES.  253 

MANNER    OF    DEPOSITION    OF    GREENALITE. 

It  now  becomes  necessary  to  determine  how  a  compound  like  g-reenalite 
can  develop  under  conditions  such  as  those  supposed  to  have  existed  in  the 
Mesabi  area.     Two  explanations  suggest  themselves. 

I.    DEVELOPMENT    SIMILAR   TO    GLAUCONITE. 

The  material  may  have  developed  in  a  manner  analogous  to  the  develop- 
ment of  glauconite.  The  manner  of  the  development  of  glauconite  is  not 
by  any  means  clear.  Perhaps  the  most  instructive  work  on  the  subject  is 
that  by  Murray  and  Renard."  Quoting  from  their  report  concerning  con- 
ditions for  the  development  of  glauconite: 

Where  the  detrital  matters  from  rivers  are  exceedingly  abundant  and  where 
there  is  apparently  a  rapid  accumulation,  glauconite,  though  present,  is  relatively 
rare;  on  the  other  hand,  along  high  and  bold  coasts  where  no  rivers  enter  the  sea, 
and  where  accumulation  is  apparently  less  rapid,  glauconite  appears  in  its  most 
typical  form  and  greatest  abundance.     *     *     * 

*  *  *  *  *  *  * 

With  reference  to  its  bathymetrical  distribution,  it  appears  to  be  most  abundant 
about  the  lower  limits  of  wave,  tidal,  and  current  action,  or,  in  other  words,  in  the 
neighborhood  of  what  we  have  termed  the  mud-line  surrounding  continental  shores. 
In  the  shallower  depths  beyond  this  line,  that  is  to  say,  in  depths  of  about  200  and  300 
fathoms,  the  typical  glauconitio  grains  are  more  abundant  than  in  deeper  water,  but 
glauconitic  casts  may  be  met  with  in  deposits  in  depths  of  over  2,000  fathoms.  No 
typical  glauconitic  sands  have,  so  far  as  we  know,  been  recorded  in  process  of 
formation  in  the  littoral  or  sub-littoral  zones.* 

Concerning  its  manner  of  development  they  state,  tentatively : 

We  are  therefore  inclined  to  regard  glauconite  as  having  its  initial  formation  in 
the  cavities  of  calcareous  organisms,  although  we  have  admitted  above  that  some 
grains  which  might  be  regarded  as  glauconite  appear  to  be  highlj^  altered  fragments 
of  ancient  rocks  or  coatings  of  this  mineral  on  these  rock  fragments.  It  appears 
that  the  shells  are  broken  by  the  swelling  out  or  the  growth  of  the  glauconite,  and 
that  subsequenfl}'  the  isolated  cast  becomes  the  center  upon  which  new  additions  of 
the  same  substance  take  place,  the  grain  enlarging  and  becoming  rounded  in  a  more 
or  less  irregular  manner,  as  in  the  case  of  concretionary  substances  like  silica,  for 
example,  which  forms  molds  of  fossils.     *     *     * 

*  *  *  After  the  death  of  the  organisms  their  shells  are  slowly  filled  with  the 
fine  mud  in  which  thej"  are  deposited.  The  existence  of  this  organic  matter  in  these 
cavities,  and  the  absence  of  all  other  causes  which  might  there  ii^duce  the  deposition 
of  the  silicates,  in  fact,  the  constant  association  of  these  phenomena  appear  to  demon- 
strate the  existence  of  a  relation  of  cause  and  effect.     *     *     *     If  we  admit  that 

«Eeport  of  the  voyage  of  H.  M.  S.  Challenger,  1S73-1878,  Deep-sea  Deposits.     ^iLoc.  cit.,  pp.  382,383. 


254  THE  MESABI  IRON-BEARING  DISTRICT. 

the  organic  matter  inclosed  in  the  shell,  and  in  the  mud  itself,  transforms  the  iron  in 
the  mud  into  sulphide,  which  may  be  oxidized  into  hj^drate,  sulphur  being  at  the 
same  time  liberated,  this  sulphur  would  become  oxidized  into  sulphuric  acid,  which 
would  decompose  the  fine  clay,  setting  free  colloid  silica,  alumina  being  removed  in 
solution:  thus  we  have  colloid  silica  and  hydrated  oxide  of  iron  in  a  condition  most 
suitable  for  their  combination.  To  explain  the  presence  of  potash  in  this  mineral 
we  must  remember  that,  as  we  have  shown  when  speaking  of  the  formation  of  palago- 
nite  under  the  action  of  sea-water,  there  is  always  a  tendency  for  potash  to  accumulate 
in  the  hydrated  silicate  formed  in  this  way.  and,  as  we  have  stated  before,  this  potash 
must  have  been  derived  from  the  sea  water." 

It  is  difficult  to  see  how  so  high  a  percentage  of  iron  as  is  found  either 
in  glauconite  or  in  greenalite^  can  be  derived  from  the  decomposition  of 
mud  filtered  into  the  interior  of  the  shell.  If  the  mud  were  derived  entirelj' 
from  the  disintegration  of  basic  rocks,  the  percentage  of  metallic  iron  would 
not  be  far  above  10  per  cent,  and  the  actual  percentage  found  in  the 
granules  is  far  higher  than  this.  The  derivation  of  sufficient  iron  from  the 
decomposition  of  mud  would  require  a  larger  amount  of  foreign  material 
than  is  contained  in  the  casts.  In  the  typical  glauconite  deposits  foreign 
material  is  present  outside  of  the  shells,  as  shown  Idv  the  above  quotation 
from  Murray  and  Renard  concerning  the  constant  accompaniments  of  glau- 
conite, and  there  seems  to  be  no  reason  why  all  of  this  material  should  not 
be  di-awn  upon  for  the  supply  of  iron.  If  the  substance  of  the  Mesabi  green 
granules  be  supposed  to  have  been  derived  from  the  decomposition  of  orig- 
inal detritus,  this  must  have  been  present  in  enormous  quantity,  for  the 
content  of  metallic  iron  shown  in  the  analyses  of  greenalite  rocks  is  25  per 
cent,  while  the  detritus  available  in  the  Mesabi  area  could  scarcely  have 
averaged  as  much  as  7  per  cent  in  metallic  iron.  As  the  greenalite  rocks  of 
the  Mesabi  iron  formation  accumulated  to  a  thickness  of  perhaps  1,000  feet, 
it  would  be  necessar}^  to  assume  that  a  thickness  of  detritus  several  times 
this  figure  originally  was  present  to  yield  the  necessary  amount  of  iron  to 
the  granules.  There  is  ample  evidence  that  no  such  amount  of  detritus  (in 
fact  little  or  none  beyond  that  now  to  be  observed)  was  ever  present  in  the 
Mesabi  iron  formation.  This  consideration  calls  for  an  additional  source 
for  the  metallic  iron  of  the  Mesabi  greenalite  granules;  other  possible 
sources  are  discussed  under  II  and  III  lielow.  The  great  thickness  of  the 
Mesabi  iron-bearing  formation  as  corajiared  ^^■ith  known  glauconite  deposits 
is  further  presumptive  evidence  tliat    processes  other  tliau  those  forming 

n  Loc.  cit.,  pp.  388,389. 


ORIGIN  OF  THE  IRON  ORES.  255 

glauconite  were  active  in  the  development  of  the  Mesabi  greenalite  gran- 
ules, for  otherwise  it  would  be  necessary  to  assume  abnormal  intensity  and 
duration  of  glauconite-forming  processes. 

While  the  explanation  of  Murray  and  Renard  as  it  stands  above 
scarcely  seems  applicable  to  the  explanation  of  the  origin  of  the  Mesabi 
granules,  it  does  not  at  all  follow  that  the  conditions  and  processes  favorable 
to  the  development  of  g'lauconite  when  these  are  fully  known  will  not  be 
applicable,  at  least  in  part,  to  the  development  of  the  Mesabi  greenalite 
granules. 

II.    DIRECT    PKECIPITATIOX    FROM    SOLUTION    BT   ORGANISMS. 

Where  iron  is  being-  contributed  to  ocean  waters  in  considerable 
abundance,  it  is  possible  to  conceive  of  minute  organisms  abstracting  the 
same  and  depositing  it  directly  in  some  such  form  as  glauconite  or 
greenalite.  This  subject  would  reauire  elaborate  treatment  and  the  expla- 
nation is  here  but  ixientioned. 

HI.    DEVELOPMENT    SIMILAR   TO    THAT   OF   IRON    CARBONATE. 

The  association  of  the  green  granules  in  the  Mesabi  iron  formation 
with  original  iron  carbonates  and  their  analogous  composition  suggest  an 
explanation  of  their  origin  similar  to  that  applied  to  the  development  of  iron 
carbonates  from  other  portions  of  the  Lake  Superior  region  by  Irving  and 
Van  Hise."     This  is  outlined  below. 

IRON  DERIVED  FROM  THE  WEATHERING  OP  PREEXISTING   KOCKS  AND  CARRIED  TO  THE  OCEAN  AS  CARBONATE. 

The  iron  and  silica  of  the  greenalite  were  brought  into  the  ocean 
largely  in  solution.  The  rocks  which  at  that  time  formed  the  shore  (the 
Archean  and  Lower  Huronian  rocks)  contained  disseminated  iron.  The 
Archean  "greenstone,"  which  formed  a  very  large  proportion  of  tlie  land 
area  at  that  time,  still  contains  from  7  to  nearly  10  per  cent  of  metallic  iron, 
largely  in  the  ferrous  condition,  and  the  Lower  Huronian  rocks  less.  By 
the  ordinary  processes  of  weathering  the  rocks  were  decomposed  and  the 
iron  taken  into  solution  by  the  surface  waters,  largely  as  carbonate.  Most 
meteoric  waters  contain  carbon  dioxide  (C0„),  and  it  is  more  than  proba- 
ble that  sulphuric  acid  was  also  present,  but  was  very  subordinate  in 
quantity  to   carbonic  acid,  and   hence   the  sulphates  were  not  important. 

a  See  Men.  U.  S.  Geol.  Survey  Vols.  XIX  and  XXVIII,  and  Twenty-first  Ann.  Kept.  U.  S. 
Geol.  Survey,  Pt.  III. 


256  THE  MESABI  IRON  BEARING  DISTRICT. 

Particles  of  the  preexisting  rocks  were  also  carried  into  tlie  ocean  in 
suspension,  and  these,  by  subsequent  decomposition,  yielded  iron  to  the 
developing  iron-formation  materials,  but  it  is  not  likely  that  this  was  a  great 
factor  in  the  development  of  the  iron  formation,  for  the  average  amount  of 
metallic  iron  in  such  particles  was  less  than  5  per  cent,  while  the  amount 
of  metallic  iron  in  tlie  original  iron-formation  rocks  is  commonly  25  per  cent. 

THE    IRON    FIRST   PRKCIPITi,TED    IN   THE   OCEAN    AS   A    HYDRATED    PEROXIDE. 

When  the  waters  bearing  iron  in  solution  reached  the  ocean  most  of 
the  iron  carbonate  was  broken  up,  the  carbon  dioxide  given  off,  and  the 
ferrous  iron  oxidized  to  a  hydrated  peroxide  state  and  precipitated.  The 
precipitate  was  probably  first  in  the  form  of  Fe„(OH)^,  although  the  degree 
of  hydi'ation  may  have  speedily  altered,  as  it  is  known  to  do  in  the 
laboratory"  when  it  is  allowed  to  stand  or  is  subjected  to  various  conditions. 
The  reaction  was 

2FeC03+0+3H,0  =  Fe,,(OH),+2CO,. 

Precipitation  may  have  occurred  either  through  the  ordinary  oxidation  or 
with  the  aid  of  "iron  bacteria."" 

THE    IRON    FIRST    PRECIPITATED    IN    ARE.AS    OP    VEGETATION. 

It  is  probable  that  the  ferric  hydrate  was  thrown  down  in  an  area  of 
abundant  vegetation.  Van  Hise  ^  has  shown  that  the  process  of  carbonation 
on  a  large  scale,  bringing  the  iron  in  form  of  carbonate  to  the  ocean,  is 
favored  by  the  presence  of  abundant  vegetation,  the  decomposition  of  which 
}delds  carbon  dioxide;  that  where  vegetation  exists  in  land  areas  it  is  also 
likely  to  be  abundant  in  adjacent  waters  where  the  conditions  allow  it ; 
and  that  in  such,  places  carbonates  are  formed.  The  very  fact  that  a 
formation  high  in  iron  content  was  deposited  in  the  Mesabi  area  shows  that' 
some  process  was  bringing  in  iron  on  a  large  scale,  and  no  process  would 
be  more  likely  to  be  normal  under  the  conditions  under  which  the  iron 
formation  developed  than  carbonation.  If  the  process  of  cai'bonation  was 
occuriing  on  a  large  scale  this  implies  the  presence  both  on  land  and  in 
water  of  abundant  vegetation.  Moreover,  the  iron  formation  contains 
original  carbon  and  carbonates  of  iron  and  calcium,  affording  direct  proof 
of  the  presence  of  vegetation  in  the  water  in  which  the  iron  ^^•as  first  pre- 

"  For  action  of  iron  bacteria,  see  any  standard  text-book  of  bacteriology. 
''Mon.  U.  S.  Geol.  Survey  on  ]Metamor|)bisin,  in  jireparation. 


ORIGIN  OF  THE  IRON  ORES.  257 

cipitated.  Still  further,  the  iron  formation,  from  its  position  between 
qnartzite  and  slate,  is  known  to  have  developed  under  conditions  inter- 
mediate between  those  of  clear  water  and  muddy  water,  which  would  be 
likely  to  be  favorable  to  vegetation.  It  has  already  been  pointed  out  that 
instead  of  the  open  ocean,  there  may  have  been  over  this  area  a  semi- 
inclosed  arm  of  the  sea.  Such  a  condition  Avould  have  been  favorable  to 
the  extensive  occurrence  of  this  process. 

THE    HYDRATED     PEROXIDE    REDUCED     BY    VEGETABLE    MATTER,     AXD    THE    PROTOXIDE    OF    IRON    COMBINED 

WITH   CARBON   DIOXIDE   OR   SILICA. 

As  the  hydrated  peroxide  fell  to  the  ocean  bottom  and  became  ming-led 
with  vegetable  material,  and  buried  with  it,  it  was  reduced,  at  least  in  part. 
Says  Van  Hise,"  "The  reducing  agent  may  be  regarded  as  carbon  monoxide, 
or  some  of  the  carburreted  hydrogens,  such  as  methane."  The  iron  was 
tlien  in  the  protoxide  state  and  could  easily  combine  with  the  carbon 
dioxide  simultaneously  developed  by  the  oxidation,  of  the  carbon  of  the 
organic  material  to  reproduce  iron  carbonate.  But  in  addition  to  the  carbon 
dioxide  developed  under  these  conditions  silica  was  present.  As  shown 
by  Van  Hise,  it  is  often  frequently  associated  with  carbonates,  and  it  was 
associated  with  carbonates  at  the  time  of  the  developinent  of  the  Biwabik 
iron  formation,  as  shown  by  its  present  occurrence  both  in  the  altered  and 
unaltered  portions  of  the  formation.  The  best  investigation  on  the  subject 
indicates  that  the  chert  is  in  most  cases  formed  through  the  agency  of 
pelagic  organisms  which  secrete  the  silica  found  in  solution  in  the  sea 
water  or  derived  from  the  decomposition  of  the  silicate  minerals  in  the 
associated  detrital  material.''  The  silica,  especially  where  in  a  colloidal 
form,  could  combine  with  the  ferrous  iron  present  to  foi-m  ferrous  silicate. 
Thus  in  the  development  of  the  Biwabik  formation  both  carbon  dioxide 
and  silica  were  present,  with  either  of  which  the  iron  protoxide  could 
combine  according  to  the  following  simple  reactions: 

FeO-t- CO3 + nH,0 = FeCO, + nH,0 
FeO+  SiO, + nH,0  -=  FeSi03nH,0 

The  last-named  formula  essentially  represents  greenalite,  though  the 
subordinate  constituents  are  not  taken  into  account. 

Whether  the  iron  combined  with  the  silica  or  with  the  carbon  dioxide 
was  probably  a  function  of  the  relative  masses  of  the  chert  and  carbon  dioxide 

«Loc.  cit.                       ^See  C.  D.  Walcott,  Mon.  U.  S.  Geol.  Survey  Vol.  XXX,  1898. 
MON  XLiir — 03 17 


258  THE  MESABl  IRON-BEAKING  DISTRICT. 

available  for  combiuation.  During  the  deposition  of  the  Mesabi  iron  forma- 
tion, silica  was  unquestionably  in  much  greater  mass  than  the  carbon 
dioxide,  as  shown  by  the  present  composition  of  both  the  altered  and 
unaltered  portions  of  the  formation.  Doubtless  there  are  other  factors  con- 
cerned in  the  combiuation  of  silica  or  carbon  dioxide  with  hon,  but  these 
are  not  vet  known.  Whatever  the  cause,  ferrous  silicate  was  formed  in 
great  abundance  and  the  iron  carbonates  in  small  quantity.  Calcium  and 
magnesium  oxides,  which  may  have  been  present,  as  shown  by  the  analyses, 
mav  have  reduced  the  active  mass  of  the  carbon  dioxide  available  to  unite 
with,  and  thus  increased  the  proportional  mass  of  silica.  Both  the  magne- 
sium and  calcium  oxides  are  stronger  bases  than  the  iron,  and  would  take 
precedence  over  it  on  going  into  combination  with  carbonic  acid. 

In  the  iron  formation  of  the  Gogebic  district  of  Micliigan,  a  forma- 
tion of  the  same  age,  general  character,  and  associations,  and  supposedly  of 
a  similar  origin,  silica  was  less  abundant  in  the  original  rock  of  the  forma- 
tion (siderite)  than  in  the  original  rock  of  the  Mesabi  iron  formation  (green- 
alite  rock),  and  thus  the  dominant  combination  was  protoxide  of  iron  and 
carbon  dioxide,  producing  iron  carbonate,  and  the  combination  of  protoxide 
of  iron  with  silica  was  very  subordinate,  although  it  did  occur,  as  shown  by 
the  small  admixture  of  ferrous  silicate  rocks  in  the  carbonates  of  this  region, 
already  noted.     (See  p.  118.) 

The  preponderance  of  iron  silicates  in  the  Mesabi  district  and  the 
preponderance  of  iron  carbonates  in  the  Penokee-Gogebic  district  suggests 
an  analogy  to  the  occurrence  of  zinc  ores  in  Missouri  and  Wisconsin, 
described  by  Van  Hise.  In  the  former  district  zinc  silicate  is  abundantly 
present  with  zinc  carbonate.  In  the  latter  district  zinc  silicate  is  sparinglv 
present  with  the  zinc  carbonate.  Van  Hise  concludes  that  "the  almost 
complete  absence  of  zinc  silicate  in  Wisconsin  and  the  presence  of  zinc 
silicate  in  southwestern  Missouri  are  in  accordance  with  the  well-known 
law  of  mass  action.  Where  silica  is  abundant,  so  that  zinc  silicate  can 
form,  it  is  jiresent  plentifully;  where  silica,  is  absent  or  subordinate,  it  does 
not  develop  in  any  considerable  quantity.""  In  a  later  work  he  states: 
"Where,  under  the  conditions  described,  silica  is  abundant  in  proper  form. 


"  Preliminary  report  on  tlie  lead  and  zinc  deposits  of  the  Ozurk  rftiion,  liy  11.  K.  Hain.  with 
an  introihu'tion  hy  C.  li.  Van  Uise;  Twenty-second  Ann.  Kt'pt.  l'.  S.  Gfol.  .-^nrvey,  Part  11,  1\H)2, 
p.  51. 


ORIGIN  OF  THE  IRON  ORES.  259 

the  law  of  mass  action  requires  its  union  with  the  protoxide  of  iron.  This 
principle  is  illustrated  by  the  condition  in  which  the  oxidized  compounds 
of  zinc  occur  in  the  Wisconsin  and  southwestern  Missouri  districts.  In  the 
Wisconsin  district  silica  is  not  especially  abundant,  and  where  zinc  sulphide 
is  oxidized  the  zinc  oxide  unites  with  carbon  dioxide  and  forms  smithsonite 
(ZnCOg)  But  in  Missouri,  silica,  partly  in  the  amorphous  form,  is  very 
abundant;  and  there,  when  the  zinc  sulphide  is  oxidized,  the  oxide  of  zinc 
largely  unites  with  the  silica,  forming  calamine  [(ZnOH)  28103].  Both 
smithsonite  and  calamine  occur  in  both  districts,  but  calamine  occurs 
abundantly  only  where  silica  is  alsundant.  Similarly,  where  in  lagoons 
the  iron  is  reduced  to  the  protoxide  form,  it  would  unite  with  the  silica  on 
a  large  scale,  j^ro^dded  that  compound  were  abundantly  present  in  a  form 
suitable  for  union."  " 

CONCLUSION   WITH  REFERENCE  TO   ORIGIN  OF  GREENALITE  GRANULES. 

It  is  concluded  that  the  explanation  offered  by  Murray  and  Renard  for  the 
development  of  the  modern  glauconite  deposits  does  not  apply  without 
much  modification  to  the  greenalite  deposits  under  discussion;  that  the 
greenalite  granules  may  possibly  have  developed  directly  from  the  abstrac- 
tion, through  the  agency  of  organisms,  of  iron  from  solution  in  sea  water, 
whence  it  was  contributed  from  adjacent  land  areas;  finally  that  a  reason- 
able explanation  seems  to  be  that  the  green  granules,  from  their  analogy 
in  composition  to  iron  carbonate,  from  their  association  with  iron  carbonate, 
from  their  great  thickness,  their  uniform  character,  and  analogy  with  other 
carbonate  and  silicate  compounds,  probably  developed  in  a  way  similar  to 
the  development  of  the  iron-carbonate  deposits  of  other  portions  of  the 
Lake  Superior  region;  and  that  their  shape,  and  not  composition,  is 
determined  by  organisms.  It  may  be  noted  in  closing  that  the  several 
explanations  of  the  origin  of  the  Mesabi  green  granules  above  outlined  are 
not  mutually  exclusive,  and  each  of  them  may  be  found  to  be  partly  true 
when  the  complete  explanation  is  known. 

«Mon.  tr.  S.  Geol.  Survey  on  Metamorphism,  in  preparation. 


260  THE  MESABl  IRON-BEARING  DISTRICT. 

BUEIAIi  OF  THE  IRON-BEAEIXG  FORIMATIOX  BENEATH  THE  VIRGrNTA 

SLATE. 

The  iron-formatiou  materials,  consisting  mainly  of  gi'eenalite  in  the 
form  of  granules,  abundant  chert  as  cementing  material  and  in  layers,  thin 
layers  of  u-on  carbonate,  and  layers  of  mud,  were  deeply  buried  under  a 
great  accumulation  of  mud,  which,  by  its  metamorphism,  has  given  the 
"\^irginia  slate  formation.  Judging  from  the  present  maximum  thictness 
of  the  slate  in  the  Penokee-Gogebic  district,  the  depth  of  burial  of  the 
Biwabik  fonnation  beneath  mud  was  thousands  of  feet  and  m.ay  have 
been  as  great  as  13,000  feet. 

EarEEGElSrCE  of  the  IEOX-BEARKS^G   FORIVIATIOX   FROM   THE  OCEA^'. 

The  iron-bearing  rocks  came  above  sea  level  by  the  uplifting  of  the 
area.  While  the  rocks  were  deeply  buried  under  j-ounger  sediments  long- 
before  the  movement  began,  induration  of  the  formation  set  in  and  con- 
tinued during  the  nplift  and,  for  parts  of  the  formation,  long  afterwards. 

ALTERATION    OF   THE   mON-BEARIXG   FOR3IATION   BIT  T\TEATHEEIXG 
AND   THE    SECONDARY    CONCENTRATION    OF   THE    ORES. 

As  soon  as  the  land  appeared  above  the  water,  erosion  began  to  wear 
down  the  rocks.  The  great  thickness  of  slate  had  first  to  be  removed,  and 
it  is  probable  that  the  iron  formation  was  not  exposed  to  weathering-  agen- 
cies until  a  very  long  period  had  elapsed.  While  part  of  the  iron  formation 
in  the  eastern  end  of  the  district  was  exposed  by  erosion  before  Keweena- 
wan  time,  as  shown  Vjv  the  fact  that  the  Keweenawan  gablDro  there  lies  on 
the  eroded  edges  of  botli  the  Virginia  slate  and  the  Biwabik  formation, 
the  central  and  western  portions  of  the  district  in  which  the  iron-oi-e  depos- 
its are  now  found  were  jirobably  not  exp<:)sed  until  post-Keweenawan  time. 
Throughout  the  Lake  Superior  region  the  Upper  Huronian  and  Keweena- 
wan rocks  were  folded  together,  and  tliere  is  reason  to  believe  that  this 
folding  developed  the  Giants  range  and  the  southward  tilting  of  the  Upper 
Huronian  strata  over  what  is  now  known  as  the  Mesabi  district.  This  fold- 
lug  also  fractured  the  brittle  iron  formation  nnd  made  it  very  pervious  to 
water.  As  soon  as  the  folding  liad  taken  ])lace  erosion  set  in,  which,  after  a 
long  j)criod,  truncated  the  edges  of  the  Upper  Huionian  series,  giving  them 
the  distribution  in  belts  parallel  to  the  Giants  range  now  to  be  observed. 


ORIGIN  OF  THE  IRON  ORES.  261 

As  the  alteration  of  the  iron  formation  and  the  concentration  of  the  ore 
has  been  brought  about  by  processes  characteristic  of.  surface  conditions, 
as  will  be  seen  on  a  subsequent  page,  the  secondary  concentration  of  the 
Mesabi  iron  ores  did  not  begin  until  after  the  truncation  following  the  post- 
Keweenawan  folding.     The  alteration  which  then  occurred  was  as  follows: 

In  its  unaltered  state  the  iron  of  the  iron  formation  was  disseminated 
through  the  rocks  of  the  formation.  Tlie  average  amount  of  metallic  iron 
was  about  26  per  cent.  The  alteration  was  rapid  at  the  surface;  but 
alteration  was  also  carried  on  below  the  surface,  both  above  and  below  the 
level  of  ground  water,  by  the  downward-percolating  siu-face  waters.  The 
bedded  structure,  the  attitiide  of  the  formation,  and  the  cross  jointing  were 
favorable  to  the  vigorous  circulation  of  waters  through  the  formation. 
Coming  down  the  south  slope  of  the  Giants  range  they  entered  the  trun- 
cated edges  of  the  iron-bearing  strata,  dipping  gently  away  from  the  range, 
and  flowed  southward  along  the  layers.  The  chief  alteration  was  that  of 
the  greenalite  which,  with  the  chert,  made  up  the  bulk  of  the  formation. 

The  waters  from  the  surface  carried  carbon  dioxide  and  oxygen  derived 
from  the  atmosphere  and  the  carbon  dioxide,  perhaps  in  part  from  tlie  erosion 
of  the  overlying  slate  and  carbonates.  They  carried  also  small  quantities 
of  sulphuric  acid  and  the  alkalies.  (See  analysis  on  p.  264.)  Hydrous  sili- 
cates are  readily  soluble  in  underground  waters,  and  especially  waters  bear- 
ing carbon  dioxide.  In  the  long  period  during  which  the  ferrous  silicate 
has  been  in  contact  with  circulating  waters  bearing  carbon  dioxide  it  is 
apparent  that  it  must  have  gone  much  into  solution.  The  iron  was  thus 
brought  into  solution  mainly  as  carbonate,  though  perhaps  partly  as  sul- 
phate or  even  as  silicate.  To  illustrate  the  essential  nature  of  the  altera- 
tion of  the  greenalite,  its  composition  will  be  assumed  to  correspond  to  the 
simple  formula  FeSiOguHaO,  thus  neglecting  the  small  amount  of  other 
substances  shown  by  the  analyses.  The  simple  ratio  of  ferrous  iron  and 
silica  shown  in  the  formula  has  not  been  proved  in  the  analyses,  but  what 
evidence  there  is  points  strongly  to  such  a  proportion.  With  this  assump- 
tion the  reaction  was 

FeSi03.H,0+H,C03+Aq=FeC03+H,SiO,+Aq 

The  iron  carbonate  was  either  immediately  oxidized  and  hydrated  and 
thrown  down  as  ferric  hydrate  through  the  agency  of  the  oxygen  carried 


262 


THE  MESABI  IRON-BEARING  DISTRICT. 


by  the  solution  which  effected  the  carbonation,  or  the  iron  carbonate  may 
have  been  in  part  .carried  a  greater  or  less  distance,  until  it  met  waters 
carrying  abundant  oxygen  and  was  then  thrown  down.  That  transporta- 
tion of  iron  in  solution  actually  occurred  is  shown  by  the  stalactitic  and 
botryoidal  ores  in  vuggs.  Where  ferrous  compounds  were  abundant  there 
was  probably  not  oxj^gen  enough  to  throw  all  of  the  ferrous  compounds  out 
of  solution  at  once.  The  oxidation  of  the  iron  carbonate  took  place  accord- 
ing to  the  following  equation : 

4  FeCOj+S  H30-t-2  0=2  FeA-3  H,,0-1-4  COa 

The  degree  of  hydration  may  have  varied  at  the  time  of  the  precipita- 
tion or  thereafter  through  varying  temperature  or  other  causes." 

The  solution  of  the  iron  silicate  (greenalite)  meant  the  simultaneous 
production  of  soluble  colloidal  silicic  acid  (see  reaction,  p.  261,  and  analy- 
sis, p  264),  which  carried  the  silica  for  greater  or  less  distances  and  by 
dehydration  de^DOsited  it  as  chert.  The  abstraction  of  the  silica  in  this 
manner  caused  the  iron  oxides  to  slump  and  thus  to  be  further  concentrated. 

This  process,  combined  with  the  actual  transportation  of  the  iron  in 
solution,  where  carried  out  on  a  large  scale,  resulted  in  the  development  of 
iron-ore  deposits;  where  on  a  small  scale  the  alteration  resulted  merely  in 
the  local  segregation  of  the  iron  oxide  and  chert,  giving  ferruginous  cherts. 

The  abstraction  of  the  silica  explains  the  slump  near  the  contact  of  the 
iron-ore  layers  with  the  layers  of  wall  rock  described  on  pages  230-233. 
Indeed,  one  would  expect  the  observed  slump  to  be  even  greater  than  it  is, 
for  the  silica  taken  out  is  over  half  of  the  volume  of  the  orig'inal  rock,  as 
may  be  noted  by  comparing  the  analyses  of  the  unaltered  rocks  and  of  the 
iron  ores.  But  the  ores  after  the  concentration  have  a  very  large  amount 
of  pore  space  compared  with  the  rocks  from  which  they  are  derived,  indi- 
cating that  the  iron  ore  has  not  fallen  to  an  extent  commensurate  with  the 
volume  of  the  silica  removed. 


«  The  effect  of  temperature  ou  the  kind  of  hydrate  precipitated  is  shown  by  the  following  analyses, 
by  Haiijpe  (Ch.  Central-Blatt,  1889,  II,  906): 


0°. 

20°. 

25°. 

30°. 

■to°. 

6O0. 

80°. 

100°. 

FeA 

54.6 
45.4 

51.4 

50.4 

46.1 
53.9 

43. 9-53.  2 


66.2 
33.8 

70.1 
29.9 

72. 3-92.  7 

Loss  on  ignition  (HjO ) 

48.6 

49.6 

ORIGIN  OF  THE  IRON  ORES.  263 

So  far  as  the  original  iron-formation  rock  was  an  iron  carbonate,  which 
we  know  to  have  been  present  but  to  have  been  very  subordinate  in  quantity 
to  the  greenahte,  a  similar  set  of  changes  occurred.  The  iron  was  oxidized 
and  hydrated  and  the  carbon  dioxide  removed  in  the  manner  described  by 
Van  Hise." 

At  the  same  time  the  slaty  layers  within  or  adjacent  to  the  iron 
formation  which  came  into  contact  with  actively  circulating  waters  were 
altered  to  paint  rock.  The  process  consisted  in  the  abstraction  of  substances 
other  than  the  silica  and  alumina  to  a  greater  extent  than  these  compounds, 
and  staining  red  with  iron  oxide,  due  in  part  to  the  oxidation  of  the  ferrous 
compounds  contained  in  the  slate  and  in  part  to  introduction  of  ferric  oxide. 

Since  the  concentration  was  started  in  post-Keweenawan  times  the 
development  of  the  ore  bodies  has  had  interruptions,  but  the  interruptions 
probably  have  been  subordinate  as  compared  with  the  long  periods  during 
which  the  concentration  has  been  occurring.  In  Cambrian  times  the  district 
may  have  been  covered  by  Cambrian  sediments,  but  this  is  uncertain.  The 
nearest  Cambrian  sediments  are  now  80  miles  distant.  Again,  in  Cretaceous 
times,  the  western  part  of  the  district,  and  perhaps  the  eastern  part,  were 
buried  to  an  unknown  depth  b}^  Cretaceous  rocks,  but  when  these  were 
stripped  ofP  by  erosion  the  concentration  of  the  ores  was  resumed. 

In  Glacial  times  the  Mesabi  district  was  oven-idden  by  the  ice  and  the 
surface  rocks  planed  off.  Large  quantities  of  iron  ore  were  removed  at  this 
time,  as  shown  by  the  fact  that  fragments  of  ore  are  found  in  the  drift,  some 
of  them  of  considerable  size.  The  so-called  Moose  Track  mine,  south  of 
the  Fayal  mine,  was  a  large  mass  of  iron  ore  of  about  30,000  tons  inclosed 
in  the  drift.  Vastly  more  ore  was  carried  away  than  now  appears  in  large 
pieces  in  the  drift,  for  the  ore  is  of  soft  character  and  would  be  likely  to  be 
finely  disseminated  through  the  drift,  or  even  carried  away  in  solution.  The 
glacial  cutting  of  the  ore  is  shown  further  b}^  the  fact  that  the  rocks  in  the 
neighborhood  of  the  iron-ore  deposits,  which  are  much  harder  than  the  ores, 
have  been  extensively  cut  down,  as  shown  by  the  abundance  of  their  de'bris 
in  the  drift;  and  as  the  surface  of  the  ores  is  now  usually  below  the  harder 
■wall  rock,  the  softer  ore  was  probabl}"  gouged  out  to  a  greater  extent  than 
the  adjacent  rocks.  If  in  the  past  the  upper  poi'tions  of  the  deposits 
contained  on  an  average  a  lai-ger  amount  of  hydi-ated  ores  than  tlie  lower 

a  Twenty-Erst  Ann.  Rept.  U.  S.  Geol.  Survey,  Pt.  Ill,  pp.  326-328. 


264  THE  MESABI  IRON-BEARING  DISTRICT. 

portions,  as  tliev  do  now,  then  the  glacial  erosion  removed  more  of  the 
highly  hydrated  ore  than  of  the  less  hydrated  ore,  and  thus  the  present 
preponderance  of  hydrated  ores  in  upper  horizons  is  not  so  great  as  it  might 
have  been  had  glacial  erosion  not  occurred. 

In  general,  the  glacial  cutting  has  probably  not  been  so  deep  as  in 
the  Canadian  iron-bearing  districts,  or  perhaps  the  Vermilion  or  Marquette 
districts,  because  of  the  protection  of  the  high  land  t()  the  north  and 
also  because  of  the  interlobate  position  of  the  district,  and  this  may  help  to 
explain  the  apparently  greater  abundance  of  ores  in  the  Mesabi  disti'ict 
than  in  the  Canadian  districts,  as  suggested  by  Van  Hise." 

Since  Glacial  time  the  ore  deposits  have  for  the  most  part  remained 
buried  beneath  glacial  drift,  but  still  the  iron  formation  has  been  under- 
going essentially  surface  alterations. 

That  the  concentration  of  the  ores  is  continuing  at  the  present  time 
is  shown  by  the  water  analysis  by  Mr.  E.  T.  Allen,  of  the  United  States 
Geological  Sm'vey,  given  below.  The  water  is  from  a  drift  between  the 
Hull  and  Rust  mines  west  of  Hibbing: 

Parts  per  lidUion. 

CO, .71 

SiO, 22. 35 

SOj 2. 2 

PO4 Trace? 

Fe Not  a  trace. 

Ca 10. 1 

K 97 

In  all  cases  except  in  the  determination  of  CO..,  ]  liter  was  used.  The  water  contained  no 
suspended  or  precipitated  matter. 

The  results  are  stated  in  terms  of  ions.  Whether  or  not  the  ionization  theory  be  accepted,  the 
radicals  given  are  the  ones  which  have  been  actually  determined  in  the  analysis,  whatever  their  state 
or  combinatioa. 

The  relatively  high  content  of  silica  in  the  water  indicates  that  this 
substance  is  now  being  taken  out  of  the  ore  deposits  and  thus  that  concen- 
tration of  the  ore  is  actually  occun-ing  at  the  present  time.  The  content  of 
carbon  dioxide,  sulphuric  acid,  and  the  alkalies  show  the  agents  to  be  present 
now  which  presumably  have  been  present  in  the  past.  The  absence  of  any 
iron  in  the  water  is  evidence  that  the  concentration  at  the  present  time 
witliin  the  ore  deposits  already  formed  consists  almost  entirely,  if  not  quite, 
in  the  abstraction  of  silica.     It  does  not  show,  liowever,  that  in  the  past 

"  Twenty-first  Ann.  Kept.  U.  S.  Geol.  Survey,  I't.  Ill,  1901, .pp.  411^12. 


ORIGIN  OF  THE  IRON  ORES.  265 

the  iron  itself  may  not  have  been  carried  in  solution  to  foi-m  these  ore 
deposits,  nor  that  iron  is  not  now  being-  carried  in  those  portions  of  the 
iron  formation!  north  of  the  Virginia  slate  and  under  the  edge  of  the 
Virginia  slate  where  greenalite  or  iron  carbonate  are  available.  At 
the  present  time  ferrous  silicate  and  ferrous  carbonate  are  in  exceed- 
ingly small  quantity  in  the  iron  formation  north  of  the  Virginia  slate, 
and  their  oxidation  takes  but  little  oxygen  from  the  percolating-  waters 
which  traverse  the  ore  bodies.  The  waters,  then,  are  likely  to  have  a 
high  percentage  of  oxygen  throughout  the  part  of  the  formation  explored 
and  the  part  from  which  the  water  analyzed  was  derived,  and  any  ferrous 
compound  which  may  get  into  solution  is  quickly  oxidized  and  precipitated. 
In  the  past,  when  the  ferrous  comjDOunds  were  abundant  where  the  ore 
bodies  now  are,  it  may  be  supposed  that  the  water  was  depleted  in  oxygen 
through  the  oxidation  of  the  ferrous  compounds  before  it  had  traveled 
any  considerable  distance  into  the  formation,  and  that  when  the  oxygen 
had  been  abstracted  the  ferrous  compounds  in  solution  may  have  been 
carried  some  distance  before  water  bearing  sufficient  oxygen  to  oxidize  and 
precipitate  them  was  encountered. 

LOCALIZATIOlSr  OF   ORES   BT   CIKCULATIOlSr  OF  WATER. 

The  segregation  of  the  iron-ore  bodies  having  occurred  through  the 
alteration  of  the  greenalite  by  the  agency  of  percolating  underground 
waters,  one  would  expect  to  find  that  the  maximum  alteration  has  occurred 
and  the  iron-ore  bodies  have  developed  at  places  where  the  circulation  of 
the  oxygen-bearing  and  carbonated  waters  was  most  vigorous.  A  consid- 
eration of  the  present  and  past  circulation  through  the  iron  formation 
shows  this  to  be  the  case. 

Meteoric  water  falling  on  the  iron  formation,  or  contributed  to  it  from 
the  slope  of  the  Giants  range  from  the  north,  enters  the  eroded  edges  of  the 
iron-formation  beds  dipping  gently  to  the  south  Water  probably  flows 
along  the  bedding  openings  more  freely  than  along  the  joints,  for  the 
bedding  openings  present  more  continuous  openings  than  the  joints,  which 
are  irregular  and  discontinuous,  and  are  frequently  cut  oif  hj  soft,  imper- 
vious layers  which  have  yielded  to  deformation  by  bending  rather  than 
by  breaking.  Locally,  however,  the  flowage  along  joints  is  dominant. 
Certain  of  the  iron-formation  layers  also  are  porous,  while  others,  particu- 


266 


THE  MESABl  IRON-BEARING  DISTRICT. 


larly  slate  and  paint  rock,  are  not,  and  thus  the  water  flows  more  easily 
through  certain  beds  than  through  others.  The  flowage  of  water  above 
the  level  of  ground  water  and  the  flowage  below  this  level  have  distinctive 
features  and  may  be  considered  separately. 

Above  the  level  of  ground  water  the  water  entering  the  iron  formation 
tends  to  move  vertically  downward  under  the  stress  of  gravity.  If  the 
substance  wei'e  homogeneous  this  tendency  would  prevail  and  the  move- 
ment would  be  practically  vertical.  Vertical  joints  in  the  iron  formation 
tend  to  allow  movement  in  this  du-ection,  but  the  more  important  bedding 
openings  and  the  combination  of  pervious  and  inij^ervious  beds  tend  to 
make  the  water  deviate  from  verticality  and  take  up  a  course  more  nearly 


Iroii--beai'ijld  iornia-tion 

Fig.  11.— Section  through  Bhvabik  formation  transverse  to  the  range,  showing  nature  of  circulation  of  water  and  its 

relations  to  confining  strata. 

parallel  to  the  beds.  The  major  flow  seems  to  follow  broad  gentle  syn- 
clines  in  the  iron-formation  layers,  but  locally  the  flow  follows  cross 
fractures. 

When  the  water  has  passed  into  the  sea  of  ground  water  its  flowage 
depends  not  only  upon  the  structure  and  texture  of  the  formation  but  upon 
the  position  of  the  outlet.  It  is  certain  that  the  water  flowing  through  the 
Biwabik  formation  does  not  circulate  vigorously  far  under  the  Virginia 
slate.  Where  the  slate  is  drilled,  in  exploration,  water  is  occasionally  found 
under  pressure,  which  shows  that,  in  spite  of  the  slight  jointing  of  the  slate, 
the  formation  is  essentially  impervious,  as  would  be  expected  from  an}- 
slate  formation,  and  has  ponded  the  water  coming  under  it  from  the  north. 
Tlius  water  flowino-  tlu-ouoh  the  iron  formation  must  therefore  necessarilv 
overflow  at  the  north  edge  of  the  slate  Recent  pumping  ojjerations  have 
lowered  the  ground  water  to  such  an  extent  that  in  places  it  stands  below 
the  slate  margin,  and  in  these  places  there  is  little  or  no  ovei-flow  at  the 
edges  of  the  slate.  In  the  past,  however,  overflow  at  this  level  must  have 
"■enerally    occurred.      Slate    layers    within    the    iron    formation    and    the 

O  m.'  »■' 

Pokefi'ama   formation   at  its   base    nmst   also   limit   the   water  circulations, 


ORIGIN  OF  THE  IRON  ORES.  267 

several  of  which  may  have  contemporaneously  existed.  Finally  the 
circulation  is  modified  by  joints.  The  general  nature  of  the  flow  of 
water  parallel  to  the  pitch  of  the  formation  between  a  slate  layer  within 
the  iron  formation  and  the  overlying  Virginia  slate,  or  between  an}^  two 
"slate  layers  within  the  iron  formation,  or  between  the  Pokegama  quartzite 
below  and  a  slate  layer  within  the  iron  formation,  is  shown  in  fig.  11,  based 
on  a  drawing  kindly  furnished  by  Prof  C.  S.  Slichter.  The  lines  of  flow 
are  drawn  on  the  assumption  that  they  enter  the  ground  at  equal  distances 
along  the  upper  slopes  and  emerg-e  in  a  limited  area  in  the  drift  near  the 
margin  of  the  slate.  Probably  more  water  enters  the  iron  foi-mation  on 
the  upper  slopes  than  on  the  lower  slopes,  being  contributed  from  the  Griants 
range  to  the  north.  Were  it  not  for  this  fact  it  would  be  necessary  to 
leave  wider  spaces  between  the  points  oi"  entrance  of  the  lines  of  flow  on 
the  upper  slopes  than  on  the  lower  slopes.  The  diagram  shows  that  the 
flow  is  more  vigorous  near  the  area  of  escape  than  elsewhere,  just  as,  when 
water  is  drawn  off  the  edge  of  a  basin,  the  flow  is  more  rapid  at  the  point 
of  escape  than  in  a  distant  portion  of  the  basin. 

The  actual  flow  through  the  iron  formation  diff'ers  from  that  shown  in 
the  diagram  in  a  number  of  particulars.  The  iron  formation  between  the 
impervious  strata  is  not  homogeneous,  but  the  attitude  of  its  layers  and 
openings  is  such  as  to  cause  little  vertical  modification  of  the  flow  shown 
in  the  above  diagram,  for  the  beds  and  bedd'ng  openings  would  tend  to 
carry  the  water  in  the  same  general  direction.  The  effect  of  heterogeneity 
is  only  to  make  the  vertical  distribution  more  uneven  than  shown  in  the 
diagram.  It  must  be  supposed  that  while  somewhat  more  water  is  coming 
to  the  surface  near  the  edge  of  the  slate  than  at  any  other  horizon  a 
considerable  quantity  of  water  is  also  coming  to  the  surface  in  the  iron 
formation  a  considerable  distance  away  from  this  horizon,  for  the  level  of 
ground  water  is  frequently  in  the  drift  above  the  rock  sui-face.  The  total 
head,  as  shown  by  the  configuration  and  attitude  of  the  water  table,  varies 
from  place  to  place  and  is  seldom  evenly  distributed  across  the  surface 
width  of  the  iron-formation  belt.  An  examination  of  the  map  will  show 
much  irreg'ularity  in  topography,  and  numerous  swamps  and  lakes, 
indicating  ground-water  level  unevenly  distributed  through  the  formation. 
Perhaps  in  general  the  head  is  greater  in  upper  portions  of  the  belt,  nearest 
the  underlining  rocks,  than  near  the  slate.  Finally,  in  the  past  the  water 
level  and  head  have  been  changed  from  time  to  time,  as  will  be  seen  below. 


268  THE  MESABI  lEON-BEARING  DISTRICT. 

Yet,  notwithstanding  these  variations  from  the  conditions  represented 
in  the  diag-ram,  it  is  thought  that  the  diagram  will  serve  as  well  as  any 
other  to  show  the  general  average  conditions  of  flow  below  water  level  in 
the  past  before  artificial  openings  had  modified  them.  It  is  qualitatively, 
if  not  quantitatively,  correct. 

The  lateral  distribution  of  the  flow  depends  upon  the  channels  avail- 
able and  the  level  of  the  points  of  escape.  Other  conditions  being  equal, 
the  flow  is  concentrated  along  joints  or  pervious  portions  of  the  formation. 
The  watej-  escapes  at  the  lowest  point,  and  hence  near  such  a  point  the 
flowage  is  likely  to  be  concentrated.  Fracturing  of  the  overlying  Virginia 
slate  or  the  slate  layers  within  the  iron  formation  may  determine  the  lowest 
point  of  escape  for  a  given  area;  or  diff'erential  subaerial  or  glacial  erosion 
may  accomplish  this  result;  or,  finally,  the  original  folding  of  the  Upper 
Huronian  series,  followed  by  the  truncation  of  the  series  by  erosion,  may 
result  in  making  the  low  points  of  escape  along  synclines.  It  is  probable 
in  many  cases  that  stream  and  ice  erosion  has  followed  the  original  struc- 
tural synclines  in  the  formation.  In  other  cases  it  is  ceilain  that  the 
anticlines  have  been  cut  oif  to  as  large  or  even  a  larger  extent  than  the 
synclines. 

While  the  flow  is  limited  below  by  impervious  strata,  if,  because  of  the 
position  of  the  outlet,  the  flow  below  the  level  of  ground  water  is  greater 
near  the  point  of  escape  than  deeper  down,  it  follows  that  there  is  no 
concentration  of  the  flow  along  the  impervious  basement,  and  thus,  in  so 
far  as  the  iron  formation  is  below  water  level,  there  is  no  reason  why  water 
circulation  should  be  more  vigorous  along  the  troug'hs  than  along  the 
arches  of  the  gentle  folds,  provided  the  openings  in  each  case  are  equal  and 
the  points  of  escape  at  equal  elevations.  If,  in  the  case  of  drawing-  off 
water  from  the  edge  of  a  basin,  above  cited,  we  assume  the  bottom  of  the 
basin  to  be  gently  flexed  instead  of  flat,  it  is  apparent  that  the  circula- 
tion will  be  more  vigorous  near  the  top  of  the  basin  than  below,  and  at  the 
l^ottom  there  will  be  practically  no  difference  in  the  circulation  over  the 
arches  and  the  circulation  over  the  troughs  so  far  as  both  are  below  water 
level.  Thus  in  the  iron  formation  below  the  level  of  ground  water,  while 
the  impervious  layers  limit  the  circulation  below,  their  shape  has  no  effect 
in  concentrating  the  flow. 

The  distriljutiou  a,nd  shapes  of  tlie   iron  deposits  and  their  relations  to 


ORIGIN  OF  THE  IRON  ORES.  269 

the  adjacent  rock  strata  are  fully  in  accord  with  the  idea  that  they  have 
been  localized  by  the  circulation  of  water  just  described.  The  history  of 
their  localization,  it  is  believed,  was  as  follows: 

When  the  formation  was  first  exposed  to  surface  alteration  meteoric 
watei's  began  to  enter  the  formation.  This  was  probably  in  earh^  Cambrian 
time,  long  before  the  Glacial  epoch,  and  the  level  of  the  ground  water 
probably  nowhere  came  to  the  rock  surface.  Indeed,  there  must  have  been 
over  the  exposed  area  of  the  iron  formation  a  belt  of  weathering  above  the 
level  of  ground  water,  comparable  in  thickness  with  that  now  observed  on 
the  upper  slopes  of  the  Giants  i-ange  where  the  drift  covering  is  not  thick. 
It  is  not  uncommon  to  find  here  a  belt  of  weathering  as  much  as  100  feet 
thick,  sufficient  to  include  the  greater  bulk  of  the  iron-ore  deposits  of  the 
range  at  the  present  time. 

In  the  belt  of  weathering  the  concentration  of  the  ores,  following  the 
circiilation  of  the  water,  was  controlled  in  its  major  distribution  by  the 
broad,  gentle  folding  of  the  formation,  but  locally  was  controlled  by  cross 
fractures.  The  ores  were  developed  along  irregular  and  ramifying  fractures 
in  the  broad  and  gentle  synclines,  and  not  uncommonly  several  more  or 
less  independent  deposits  were  developed  in  the  same  syncline.  As  noted 
on  a  prior  page,  under  these  circumstances  the  occurrence  of  the  ores  in 
synclines  is  frequently  not  directly  shown  by  the  attitude  of  the  iron- 
formation  layers  immediately  adjacent  to  the  deposits,  for  the  fractures 
along  which  the  ores  have  developed  may  cross  any  part  of  the  syncline 
and  thus  intersect  layers  with  almost  any  attitude.  Alteration  once  begun 
in  any  area,  the  abstraction  of  the  silica  made  the  rocks  porous  and  tended 
to  confine  further  circulation  and  consequent  alteration  to  the  same  area. 
That  the  concentration  of  the  ore  deposits  has  occurred  along  cross 
fractures  is  shown  not  only  by  their  distribution  and  shape,  but  by  the 
fact  that  the  contacts  of  the  ore  with  the  wall  rock  or  with  horses  of  rock 
within  the  deposit  are  plane  surfaces  intersecting  at  various  angles,  and 
when  the  ore  has  been  removed  the  rocks  stand  out  in  castellated  forms. 
The  verj'  existence  of  horses  of  rock  in  the  ore  deposit  is  evidence  of  the 
concentration  along  cross  fractures  which  leave  intermediate  rock  masses. 
Still  further  showing  the  control  of  circulation  by  fractures  is  the  fact 
that  many  hand  specimens  may  be  collected  showing  plane  surfaces  with 
alteration  extending  a  little  way  in  from  the  surface,  indicating'  the  agent 


270  THE  MESABl  IRON-BEARING  DISTRICT. 

of  the  alteration — water — to  have  followed  this  surface.  In  general,  the 
expression  sometimes  used  locally  on  the  range  that  the  ore  results  from 
"rotting"  along  fractures  may  be  close  to  the  truth,  although  requiring 
certain  modifications,  as  above  indicated. 

When  the  water  passed  below  the  level  of  ground  water  it  continued 
its  work  of  alteration,  though  perhaps  not  so  vigorously  because  of  its 
depletion  in  oxygen  and  carbon  dioxide.  But  while  the  concentration  was 
limited  below  by  the  slate  layers  of  the  iron  foriiiation,  the  concentration  of 
the  ores  was  not  necessarily  confined  to  openings  in  the  broad  synclines. 
In  this  part  of  its  course  the  water  concentrated  ores  along  the  largest  and 
most  continuous  openings  and  near  the  lowest  points  of  escape,  regardless 
of  the  shape  of  the  impervious  basement.  But  in  so  far  as  the  lowest  point 
of  escape  was  determined  primarily  by  the  folding,  as  it  doubtless  was  to  a 
great  extent,  the  concentration  occurred  along  synclines  in  the  impervious 


Boundaries  of  /ron  formation 
Erosion  surface 

■  ore  deposit  ° ?520 10eO_ 


Fig.  12.— Sketch  shoiving  three  stages  in  the  downward  and  lateral  migration  of  an  ore  deposit  due  to  the  truncation  of 

the  iron  formation  by  erosioji. 

basement.  This  is  illustrated  by  the  Biwabik  mine.  The  ore  deposit  is  in 
general  parallel  to  the  strike  of  the  iron-formation  strata,  liut  its  thickest 
portion  may  be  seen  to  lie  near  the  lower  point  in  the  paint  rock  (the 
altered  equivalent  of  the  Virginia  slate  at  this  point)  which  may  be  seen  to 
overlie  the  ore  body.  Also  exploration  to  the  south  shows  the  slate  to  be 
cut  back  just  opposite  the  "belly"  of  the  deposit. 

As  the  process  of  erosion  continued  the  surface  of  the  iron  formation  was 
cut  down,  and  this  was  accompanied  by  a  migration  downward  of  the  le^•el 
of  ground  water,  with  the  consequent  emergence  above  this  level  of  the  part 
of  the  iron  formation  which  had  heretofore  been  below  ground  water.  Any 
concentration  which  had  occurred  before  this  part  of  the  formation  came 
above  water  level  was  probably  continued  in  the  same  areas,  for  such  areas 
were  rendered  more  jiorous  by  the  alteration  already  undergone,  but  any 
new  concentration  was  along  the  impervious  basements  of  pitching  troughs. 

The  erosion  of  the  surface  of  the  iron  formation  resulted  in  the  lateral 


ORIGIN  OF  THE  IRON  ORES.  271 

downward  migration  of  the  ore  deposits.  As  the  ore  was  cut  off  above,  the 
development  of  new  ore  continued  downward  along  the  dip,  its  lowest  limit 
always  marked  by  an  impervious  stratum.  Tliree  hypothetical  stages  of 
the  development  of  the  ore  dejDOsit  and  its  lateral  migration  are  shown  by 
fig.  12  (p.  270).  It  is  apparent  from  this  that  the  present  state  of  affairs  is 
merely  a  stage  in  a  contii:iuous  process  of  concentration  and  migration, 
which  is  continuing  to-day  as  it  has  in  the  past. 

There  is  evidence  that  the  area,  before  Grlacial  time,  may  have  been 
near  the  sea  level  in  two  periods,  the  Cambrian  and  Cretaceous.  A.t  these 
periods  the  level  of  ground  water  must  have  been  near  the  rock  surface; 
the  circulation  was  feeble  and  the  concentration  of  the  ore  was  slow  and 
possibly  ceased.  At  intermediate  and  subsequent  times  the  level  of  ground 
water  may  be  supposed  to  have  been  some  distance  beneath  the  surface. 
The  circulation  was  then  vigorous  and  the  development  going  on. 

In  Grlacial  times  a  considerable  portion  of  the  upper  part  of  the  forma- 
tion was  scraped  off.  This  included  a  large  part  of  the  formation  which 
had  been  above  the  level  of  ground  water,  and  thus  many  of  the  ore 
deposits  and  parts  of  ore  deposits  which  had  been  concentrated  in  this 
belt.  Associated  with  the  glacial  cutting  was  the  deposition  of  a  thick 
mantle  of  glacial  drift,  into  which  the  ground  water  worked  up.  Since 
glacial  time  a  large  part  of  the  iron  formation  has  been  below  the  level  of 
ground  water. 

In  summary,  the  localization  of  the  ores  through  the  circulation  of 
underground  water  has  been  controlled  primarily  by  the  broad,  shallow 
synclines"  into  which  the  iron-formation  layers  have  been  folded,  but  other 
factors  have  greatly  modified  this  control  and  locally  have  been  dominant. 
Of  the  modifying  factors  the  principal  one  has  been  the  cross  fracturing  of 
the  iron  formation,  yielding  openings  through  which  the  waters  have  flowed 
in  devious  paths  and  causing  the  concentration  of  the  ores  in  limited  and 
irregular  areas  within  the  synclines.  Of  scarcely  less  importance  have  been 
the  little  fractured  and  relatively  impervious  slate  layers  within  the  iron 
formation  and  the  Virginia  slate  above  the  iron  formation,  limiting  the  cir- 
culation above  and  below,  ponding  the  underground  water,  causing  lateral 

«  Such  synclines  are  not  necessarily  surface  troughs,  aa  sometimes  assumed  by  explorers.  They 
are  evidenced  by  the  attitudes  of  the  layers  of  the  iron  formation,  and  may  not  be  apparent  in  the 
unequally  eroded  rock  surface  or  at  the  surface  of  the  irregular  covering  of  glacial  drift. 


272  THE  MESABI  IRON-BEARING  DISTRICT. 

movements  toward  the  lowest  points  of  overflow,  and  finally  causing  the 
flow  to  be  not  necessarily  confined  to  synclines  but  in  some  cases  to  be 
vigorous  over  anticlines,  all  combining-  to  explain  the  concentration  of  iron- 
ore  deposits  with  greater  dimensions  parallel  to  the  strike  of  the  iron- 
formation  layers  (parallel  to  the  trend  of  the  range),  and  deposits  apparently 
independent  of  any  synclinal  structure  in  the  iron  formation. 

EXPLAKATIOIS"    OF    THE  APPARENT   ABSEKCE    OF    ORE    DEPOSITS   AT 
THE    EAST    EXD    OF    THE    RANGE. 

Eastward  from  Mesaba  station  in  the  proximity  of  the  Duluth 
gabbro  the  iron  formation  has  no  known  ore  deposits.  The  iron  oxide 
present  is  mainly  magnetite  instead  of  hydrated  hematite,  and  the  asso- 
ciated minerals,  aside  from  the  chert,  are  characteristically  monoclinic 
amphibole.  These  conditions  indicate  a  history  for  this  part  of  tlie  district 
somewhat  different  from  that  above  described  for  the  part  bearing  the  ore 
deposits. 

The  iron  formation  in  the  east  end  of  the  district  was  originally  similar 
in  character  to  that  in  the  western  and  central  portions  of  the  district,  though 
perhaps  containing  a  little  more  intercalated  slate.  While  the  iron  forma- 
tion of  the  central  and  western  portions  of  the  district  probably  was  not 
exposed  to  surface  alteration  until  post-Keweeuawan  time,  in  the'  eastern 
end  of  the  district  the  iron-formation  rocks  were  exposed  by  the  removal 
of  the  overlying  .slate  before  Keweenawan  time.  In  Keweenawan  time 
the  eastern  end  of  the  district,  and  perhaps  the  central  portion,  were  buried 
beneath  Keweenawan  rocks,  principal  among  which  was  the  Duluth  gabbro 
mass  of  northeastern  iMinnesota."  In  the  east  end  of  the  district  the  gabbro 
came  to  rest  on  the  eroded  edges  of  the  iron  formation  and  slate.  In  the 
central  and  western  ends  of  the  district  the  gabbro  was  separated  from  the 
iron  formation  by  a  vast  thickness  of  slate  which  up  to  that  time  had  not 

<i  If  the  iron  formation  had  been  exposed  prior  to  the  advent  of  the  gabbro,  and  normal  Hmonites 
or  hematite.?  formed  to  some  extent,  one  might  expect  to  find  tliat  tlie  gabbro  had  metamorphosed 
these  oxides  into  hard,  Ijrilliantly  colored  jaspers.  This  is  apparently  the  explanation  of  certain  of 
the  jaspers  in  the  Gogebic  district.  No  such  jasj)ers  have  been  found  in  the  INIesabi  district,  and  their 
al>^:■euce  might  be  taken  as  evidence  that  the  iron  formation  had  not  been  exposed  at  the  east  end  of  the 
district  prior  to  the  intrusion  of  the  gabbro.  This  evidence,  however,  it  is  believed  will  not  stand  against 
the  evidence  of  the  structural  relations  of  the  gabbro  to  the  iron-formation  rocks  described  on  pp. 
197-198.  The  lack  of  jaspers  may  rather  indicate  that  the  alteration  under  surface  conditions  prior  to 
Keweenawan  time  ha<l  scarcely  more  than  begiin  when  the  gabbro  wa.-^  intruded. 


ORIGIN  OF  THE  IRON  ORES.  273 

been  removed  by  erosion.  The  iron  formation  of  the  eastern  end  of  the  dis- 
trict, which  had  been  exposed  to  surface  weathering  for  a  short  time,  was  at 
tlie  advent  of  the  gabbro  brought  under  deep-seated  conditions  of  alteration. 
When  thus  buried  and  receiving  heat  from  the  cooHng  of  the  gabbro,  the 
alterations  were  those  of  partial  oxidation,  perhaps  deoxidation,  dehydration, 
decarbonation,  and  silication,  characteristic  of  such  conditions. 

The  greenalite  was  altered  without  the  presence  of  sufficient  oxygen 
to  bring  the  iron  up  to  the  ferric  state,  and  the  result  was  the  development 
of  magnetite,  which  may  have  gone  through  an  intermediate  hydrated  stage. 
A  temperature  of  only  19°  C.  is  necessar^^  to  dehydrate  the  hydrate  of  the 
ferrous-ferric  oxides.  There  is  a  possibility  also  that  the  magnetite  may 
have  been  developed  to  some  extent  at  this  time  by  the  dehydration  and 
deoxidation  of  an}^  small  amount  of  ferric  hydrate  which  may  have  existed 
af  this  time.  Dehydration  alone  of  the  fer)-ic  hydrate  maj'  have  also 
occurred, 

A  very  characteristic  change  during  Keweenawan  time  was  the  devel- 
opment of  amphiboles.  Griinerite,  FeSiOs,  actinolite,  cummingtonite,  and 
perhaps  other  amphiboles,  developed  from  greenalite  by  dehydration  and 
the  redistribution  of  the  calcium,  magnesium,  iron,  and  silica  in  the  granules 
and  matrix.  Where  the  orig'inal  material  was  carbonate,  as  it  was  to  a 
limited  extent,  the  development  of  the  amphiboles  involved  decarbonation 
and  silication,  the  carbonate  being  carried  away  in  solution. 

Still  another  alteration  characteristic  of  the  Keweenawan  was  the 
marked  recrystallization  of  the  chert  and  the  matrix,  resulting  in  a  great 
coarsening-  of  the  grain.     (See  pp.  159-160  and  PI.  XVII.) 

That  the  Keweenawan  intrusion  and  burial  were  the  main  causes  of 
the  development  of  magnetite  and  the  amphiboles  is  shown  by  the  present 
distribution  of  these  minerals,  which  are  most  abundant  near  the  gabbro 
and  decrease  in  amount  as  the  distance  from  the  gabbro  increases,  until  in 
the  central  and  western  portions  of  the  district  they  disappear  almost 
entirely.  Also  near  the  gabbro  contact  the  grain  of  the  chert  is  the  coarsest, 
and  in  leaving  the  gabbro  the  grain  becomes  finer. 

Erosion  subsequent  to  Keweenawan  time  imcovered  a  large  portion  of 
the  iron  formation  of  the  district  and  brought  the  formation  under  conditions 
of  surface  alterations.  The  magnetitic,  the  amphibolitic,  and  coarsely  crys- 
talline rocks  of  the  eastern  portion  of  the  district  were  very  resistant  to  such 

MON  XLIII — 03 18 


274  THE  MESABI  IRON-BEARING  DISTRICT. 

alterations,  in  this  differing  from  the  iron-formation  rocks  of  the  central  and 
western  portions  of  the  district,  which,  being  soluble  ferrous  silicates,  when 
first  exposed  to  surface  alterations  were  quickly  altered.  Thus  the  rocks 
at  the  east  end  of  the  range  have  remained  practically  as  left  by  the  gabbro. 
If  there  has  been  no  further  concentration  of  the  iron  in  the  eastern  portion 
of  the  range  since  Keweenawan  time,  the  amount  of  magnetite  there  to  be 
observed  measures  the  amount  of  concentration  which  had  occurred  in  the 
tbrmation  when  left  by  the  gabbro.  As  already  noted,  magnetite  has  thus 
far  not  been  found  to  be  concentrated  into  workable  deposits. 

CAUSE    OF   THE    DISTRIBUTION    OF   PHOSPHORUS. 

Phosphorus,  while  locally  very  irregular  in  distribution,  in  general 
is  more  abundant  in  certain  phases  of  the  iron  formation  than  in  others. 
The  paint  rocks  contain  the  highest  percentage  of  phosphorus.  These 
are  followed  in  order  by  the  ^^ellow  ores,  the  blue  and  brown  ores,  the 
ferruginous  cherts,  the  unaltered  slates,  and  the  unaltered  greenalite 
rocks.  The  last  named  contain  little  or  none  of  this  element.  As  the  iron 
ores  and  the  ferruginous  cherts  have  resulted  from  the  alteration  of  the 
greenalite  rocks,  and  the  paint  rocks  have  resulted  from  the  alteration  of 
the  slates,  it  is  apparent  that  the  alteration  has  been  accompanied  by  the 
introduction  of  phosphorus;  in  other  words,  that  the  phosphorus  in  the  ores 
and  associated  rocks  has  been  mainl}^  introduced  from  without  and  is  not  a 
residual  product  derived  from  the  decomposition  of  the  original  rocks  of  the 
formation.  Almost  any  of  the  rocks  outside  of  the  iron  formation  contain 
phosphorus,  and  some  of  them — for  instance,  the  granite  of  the  Giants 
range — contain  more  than  is  known  in  any  of  the  rocks  of  the  iron  formation. 
Waters  entering  the  iron  formation,  therefore,  carry  small  quantities  of 
phosphorus  derived  from  the  decomposition  of  such  rocks.  The  amount 
may  be  small  (the  analysis  of  water  on  p.  264  shows  but  a  trace);  yet  the 
amount  of  phosphorus  in  the  ores  and  the  adjacent  rocks  is  usually  a  small 
fraction  of  1  per  cent,  and  when  we  consider  the  vast  time  during  which 
the  concentration  has  been  occurring,  it  is  apparent  that  but  an  exceedingly 
small  amount  of  phosphorus  need  be  in  solution  at  any  one  time.  The 
phospliorus  is  probably  caiTied  in  solution  as  a  phosphate. 

The  precijjitation  is  believed  to  be  occasioned  by  some  aluminous 
compound.  The  paint  rock,  which  has  the  highest  content  of  phospliorus, 
has  also  a  high  content  of  alumina.     Aluminum  phosj)hate  is  insoluble,  and 


ORIGIN  OF  THE  IRON  ORES.  275 

this  suggests  the  possibihty  that  the  aluminous  mineral  may  be  the  precipi- 
tating agent.  Some  of  the  yellow  ores  also  contain  more  alumina  than  the 
brown  and  blue  ores  (in  part  because  of  their  content  of  paint  rock), 
although  it  is  doubtful  whether  any  general  statement  that  these  ores  con- 
tain liigh  content  of  alumina  can  be  made.  So  far  as  the  yellow  ores  do 
contain  more  alumina  than  the  blue  or  brown  ores,  their  greater  content  of 
phosphorus  would  be  in  accord  with  the  supposition  that  the  alumina  is  the 
precipitating  agent.  Yet  the  phosphorus  content  is  also  high  in  yellow  ores 
which  probably  do  not  have  a  higher  alumina  content  than  the  brown  or 
blue  ores.  For  instance,  in  the  Oliver  mine,  in  1899,  a  vein  of  limouite 
-  could  be  seen  cutting  down  from  the  surface,  clearly  as  the  result  of  an 
alteration  by  percolating  waters  along  a  fissure,  and  the  percentage  of  phos- 
phorus within  the  vein  was  much  higher  than  in  the  ore  immediately  adjacent. 
As  alumina  is  very  insoluble  and  not  likely  to  be  infiltrated  along  a  fissure, 
it  is  not  likely  that  the  percentage  of  alumina  in  this  vein  is  higher  than  in 
the  adjacent  ore. 

It  is  not  imlikely,  also,  that  some  of  the  phosphorus  in  the  Mesabi  ores 
occurs  in  the  form  of  apatite.  Prof  A.  E  Seaman,  of  the  Michigan  School 
of  Mines,  has  found  apatite  crystals  in  the  ores  of  the  Michigan  iron  ranges, 
and  it  seems  reasonable  to  suppose  that  they  may  also  be  found  in  the 
Mesabi  oi-es,  although  the  writer  has  thus  far  hunted  for  them  without  suc- 
cess. Even  if  the  phosphorus  is  present  partly  or  wholly  as  apatite,  it  is 
still  believed  that  the  constant  association  of  phosphorus  with  alumina 
indicates  that  aluminous  compounds  are  essential  to  the  reactions  precipi- 
tating the  phosphorus  compounds.  It  may  be  noted  in  this  connection  that 
in  the  Michigan  iron  ranges,  where  Professor  Seaman  has  found  the  apatite,, 
the  close  association  of  the  phosphorus  and  alumina  seems  also  to  hold. 

An  interesting  characteristic  which  the  phosphorus  compounds  in  the 
Mesabi  ores  possess  in  common  with  those  in  the  Michigan  ores  is  their 
solubility.  When  allowed  to  stand  in  the  stock  pile  they  are  easily  dis- 
solved out.  In  the  Biwabik  mine  ore  shipped  from  the  east  stock  pile  was; 
found  to  run  .036  to  .045  in  phosphorus.  When  stock  piled  three  years 
previous  the  phosphorus  content  varied  from  .050  to  .065  and  averaged 
about  .060.  Thus  in  three  years  the  leaching  out  of  phosphorus  by  ordi- 
nary meteoric  agencies  was  sufficient  to  turn  a  distinctly  non-Bessemer 
grade  into  a  Bessemer  grade. 


'i7()  THE  MESABI  IRON-BEARING  DISTRICT. 

x'OINTS  OF  SIMILARITY  AKD  DIFFERENCE  BETWEEN  MESABI  ORES 
AND  THOSE  OF  OTHER  liAKE  SUPERIOR  RANGES. 

The  iron-ore  deposits  of  the  Mesabi  are  similar  to  those  of  the  other  Lake 
Superior  ranges,  commonly  known  as  the  "old  ranges, "  in  the  following  features: 

1.  They  are  concentrates  from  an  original  ferrous  iron  compound. 

2.  The}'  have  been  concentrated  by  surface  waters  bearing  carbon 
dioxide  and  oxygen. 

3.  They  are  located  where  the  circulation  of  waters  has  been  most 
vigorous. 

4.  They  are  mainly  in  pitching  basins,  at  least  in  part  bottomed  by 
impervious  strata. 

The  Mesabi  iron-ore  deposits  differ  from  those  of  the  old  ranges  in  the 
following  features: 

1.  The  original  material  which  by  its  alteration  has  given  the  iron  ore 
in  the  Mesabi  district,  while  a  ferrous  compound  of  iron,  is  mainly  a  silicate 
rather  than  a  carbonate. 

2.  The  pitching  basins  in  the  old  ranges  are  formed  by  a  folded  imper- 
Anous  formation  or  by  the  intersection  of  the  impervious  formation  with  a 
dike,  as  in  the  Penokee-Gogebic  district.  The  ore  lies  against  the  impervious 
wall  rock  at  the  bottom  and  also  to  a  considerable  extent  at  the  sides.  In 
the  Mesabi  district  the  ores,  while  largely  confined  to  broad,  gentle  syn- 
clines,  are  in  in-egular  bodies,  occupying  but  a  small  part  of  the  syncliue, 
and  the  local  attitude  of  the  iron-formation  layers  immediately  adjacent  to 
the  ores  may  give  no  indication  of  a  synclinal  structure. 

3.  The  Mesabi  ore  deposits  are  for  the  most  part  shallow  and  have 
great  hoi'izontal  dimensions,  while  those  of  the  old  ranges  are  deep  and 
narrow.  The  Mesabi  deposits  are  seldom  deeper  than  300  feet,  and  mav 
have  a  considerably  greater  horizontal  extent — in  some  cases  more  than  a 
mile.  Deposits  of  the  old  ranges  may  reach  a  depth  of  2,000  feet  or  more, 
and  seldom  liave  a  surface  extent  gi-eater  than  a  few  lumdred  feet.  On  the 
Mesabi  range  the  areal  extent  of  the  ore  dejiosits  is  about  5  per  cent  of  the 
entire  iron  formation,  wliile  in  the  old  ranges  the  percentage  is  less. 

4.  'I'lie  ])itch  of  tlie  deposits  is  unifornih'  less  steep  in  tlie  ]\Iesahi  range 
than  in  tlie  old  ranges.     Average  pitches  range  from  10"  to  20°. 

f).  I'he  Mesabi  ores  are  niiich  more  liydrated  than  the  ores  of  the  old 
ranges,  with  the  possible  exception  of  those  of  the  Gogebic  range,  and  they 
five  on  an  average  of  a  softer  character.     Tlie  silica  content  also  averages  less. 


ORIGIN  OF  THE  IRON  ORES.  277 

PREVIOUS  EXPLAlSrATIOlVS  OF  THE   ORIGIN  OF  THE   ORE  A^ND  THEIR 
RELATIONS   TO   THE  EXPIiANATIOjS^  ABOVE   GIVE?4^. 

Explanations  of  the  origin  of  the  ore  on  the  south  shore  of  Lake 
Superior  were  early  made  by  Hubbard,  Rivot,  Kimball,  Hunt,  Brooks, 
Wright,  Houghton,  and  Locke,  but  it  was  not  until  1869  that  the  carbonate 
theory  now  generally  accepted  was  suggested.  In  that  year  Credner  made 
the  suggestion,  with  special  reference  to  the  Marquette  district  of  Michigan, 
that  the  ores  were  derived  from  the  alteration  of  iron  carbonate " 

In  1886  Irving^  concluded  that  the  iron-formation  rocks,  including  the 
iron  ores,  of  the  entu-e  Lake  Superior  region  were  derived  from  the  alteration 
of  an  iron  carbonate.  This  conclusion  was  based  primarily  on  work  in  the 
Penokee-Gogebic  district,  but  other  iron-bearing  districts,  including  the 
Gunflint  area,  had  been  examined  for  corroborative  evidence. 

In  1889"  and  1892"*  Van  Hise,  who  had  done  much  of  the  field  and 
petrographic  work  on  which  Irving  based  his  conclusions  as  to  the  origin 
of  the  Penokee-Gogebic  iron-formation  rocks,  presented  detailed  proof  of 
the  alteration  of  the  Penokee-Gogebic  iron-formation  rocks  from  iron 
carbonate.  In  addition  he  showed  the  occurrence  of  the  ores  in  pitching 
troughs  with  impervious  basements  where  percolating  waters  are  converged, 
and  applied  the  long-accepted  ideas  of  the  manner  of  formation  of  iron 
carbonate  to  the  iron  carbonate  in  the  Penokee-Gogebic  district.  Since 
1892  Van  Hise  and  his  assistants  have  applied  essentially  the  same 
explanation  to  the  origin  of  the  iron-bearing  rocks  and  ores  of  the 
Marquette,  Crystal  Falls,  Menominee,  Vermilion,  and  in  part  to  the 
Michipicoten  districts,*  the  only  exception  being  the  Groveland  formation 
of  the  Felch  Mountain  area,  which  Smyth-''  thought  to  have  developed 
from  the  alteration  of  glauconite  rocks,  in  this  adopting  Spurr's  explanation 
of  the  origin  of  Mesabi  ores  noted  below. 

Little  or  nothing  was  known  of  the  Mesabi  hematite  ores  until  1891, 

a  Die  vorsilurischen  Gebilde  der  "Oberen  Halbinsel  von  Michigan"  in  Nord-Ainerika,  by  H. 
Credner:  Zeitschr.  deutsch.  geol.  Gesell.,  Vol.  XXI,  1869,  p.  547. 

^Origin  of  the  ferruginous  schists  and  iron  ores  of  the  Lake  Superior  region:  Am.  Jour.  Sci.,  3d 
series,  Vol.  XXXII,  1886,  pp.  2,55-272. 

cAm.  Jour.  Sci.,  3d  series.  Vol.  XXXVII,  1889,  pp.  32-48. 

<^Mon.  U.  S.  Geol.  Survey  Vol.  XIX. 

tiMon.  U.  S.  Geol.  Survey  Vols.  XXVIII,  XXXVI,  and  XLV;  Folio  62;  Twenty-first  Ann. 
Kept.  Pt.  III. 

.r'Mon.  U.  S.  Geol.  Survey  Vol.  XXXVI,  pp.  421-422. 


278  THE  MESABI  IRON-BEARING  DISTRICT. 

although  the  magnetites  at  the  east  end  of  the  range  had  Ijeen  known  ]ong 
before.  In  1886  Irving"  had  stated  that  the  Mesabi  ores  were  devehiped 
from  the  alteration  of  iron  carbonate,  but  this  statement  was  based  on  scanty 
observations  made  to  the  east  of  the  Mesabi  range,  at  Gunflint  Lake  and 
vicinity.  N.  H.  Winchell,''  in  1891,  concluded  that  the  Mesabi  ores  were 
formed  by  chemical  precipitation  as  hydrated  sesquioxide  in  the  Taconic 
ocean.  In  1893  H.  V.  Wiuchell"  first  noted  the  occurrence  of  the  Mesabi 
ores  in  pitching  troughs,  with  the  Pokegama  quartzite  as  the  impervious 
basement,  where  they  were  deposited  by  percolating  waters  flowing  along 
natural  underground  drainage  lines.  The  ores  were  supposed  to  result  from 
the  alteration  of  some  one  type  of  rock,  for  the  most  part  not  a  carbonate, 
which  he  had  not  discovered,  but  from  the  associations  and  character  of  the 
formation  thought  probably  to  be  a  bedded  oceanic  chemical  ]n'ecipitate. 
Spurr''  in  1894,  by  detailed  microscopic  work,  determined  the  original  rock 
of  the  iron  formation  to  be  made  up  of  green  granules  in  a  matrix  con- 
sisting essentially  of  chert.  The  green  granules  were  found  to  be  essentially 
ferrous  silicate  and  to  have  many  of  the  physical  properties  of  glauconite. 
He  concluded,  therefore,  that  they  are  glauconite  deposited  through  organic 
agencies  in  a  sedimentary  succession,  and  that  the  iron  carbonates  present 
in  the  iron  formation  ai'e  probably  secondary  to  glauconite.  The  develop- 
ment of  the  iron  ores  from  the  gi'een  granules  was  emphasized.  He 
followed  H.  V.  Winchell  in  noting  the  concentration  of  the  ore  bodies  by 
percolating  waters  upon  the  impei'vious  Pokegama  quartzite,  and  supposed 
the  waters  to  have  followed  planes  of  weakness  whether  this  happened  to 
be  along  troughs  or  arches.  Major  faults  of  considei-able  displacement 
were  supposed  to  account  for  the  localization  of  many  deposits.  Spurr's 
explanation  of  tlie  development  of  the  Mesabi  ores  from  glauconite  rocks 
was  accepted  by  N.  H.  Winchell''  until  1900,  when  he  announced  that  the 
oreen  ffraiuxles  were  volcanic  sand. 

Spurr's  conclusion  that  the  ores  have  developed  from  the  alteration  of 
green  granules,  consisting  essentially  of  ferrous  silicate,  has  been  fully 
confirmed  bv  the  work  done  in  prej)aration  for  this  monograjih:  his  conclu- 
sion that  the  green  granules  ai'e  glauconite  grains  has  not  been  accepted; 

"Loc.  cit. 

''Geol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  6,  1891. 
•    ^Twentieth  Ann.  Kept.  Geol.  Nat.  I-li,*t.  Survey  Minnesota,  ISflS. 
(I Geo].  Nat.  Hint.  Survey  Minne.sota,  Hull.  No.  10,  1894. 
«Geol.  Nat.  Hist.  Survey  Minnesota,  Final  Rc'iit.,  N'ul.  V,  liiiio.  p]..  iWi)-999. 


ORIGIN  OF  THE  IRON  ORES.  279 

his  conclusion  that  the  iron  carbonate  is  entirely  secondary  to  glauconite  has 
not  been  followed — some  of  it  is  secondary,  but  much  of  it  also  is  unques- 
tionably orig'inal  and  correlative  in  origin  with  the  green  granules;  and, 
finally,  his  conclusion  as  to  faulting'  in  the  area  and  the  relation  of  faults  to 
the  localization  of  ores  is  not  borne  out  by  the  present  work. 

In  1901  samples  of  the  Mesabi  green  gtanules  were  submitted  by  the 
writer  to  the  chemists  of  the  United  States  Greological  Survey  and  were 
found  to  have  a  composition  indicating  them  not  to  be  glauconite.  On  the 
basis  of  these  results,  Professor  Van  Hise,  in  his  report  on  the  iron  ores  of 
the  Lake  Superior  region  in  the  Twenty -first  Annual  Report  of  the  United 
States  Geological  Survey  (1901),  pointed  out  the  analogy  in  composition 
between  the  substance  of  the  green  granules  and  iron  carbonate  and  their 
probably  analogous  development.  Since  the  publication  of  this  report,  fur- 
ther analyses  of  the  substance  by  chemists  of  the  United  States  Geological 
Survey,  discussed  in  the  present  monograph,  have  confirmed  these  results. 

In  the  present  report  also  attention  has  been  paid  to  the  cause  of  the 
occurrence  of  greenalite  in  granules  and  to  the  similarity  not  only  to  glau- 
conite grains  but  to  the  organic  deposits  of  the  Clinton  ores,  and  the  con- 
clusion is  reached  that  the  shapes  of  the  granules  may  be  essentially  due  to 
accretion,  replacement,  filling,  or  any  combination  of  them,  about  organic 
bodies.  Thus  Spurr's  conclusion  that  the  granules  owe  their  shapes  to 
organisms  is  retained  with  modifications. 

In  the  Twenty -first  Annual  Report  cited,  the  development  of  the  ore 
deposits  by  concentration  through  the  agency  of  percolating'  waters  was 
noted  by  Van  Hise  and  the  writer,  and  the  circulation  of  the  water  was 
supposed  to  be  controlled  by  the  gentle  cross  folds  in  the  iron  formation, 
emphasized  in  the  ore  deposits  by  the  slump  due  to  the  extraction  of  silica, 
by  impervious  slaty  layers  within  the  iron  foriaiation,  by  the  overlying 
Virginia  slate,  and  finally  by  planes  of  weakness  formed  by  fracturing 
within  the  troughs. 

In  the  jDresent  monograph  emphasis  is  placed  on  the  fact  that,  while 
the  primary  control  of  the  circulation  and  consequent  concentration  has 
been  exercised  by  the  broad  shallow  synclines  in  the  iron  formation,  other 
factors,  such  as  fracturing,  slate  .layers  within  the  iron  formation,  and  the 
overlying  Virginia  slate,  have  greatly  modified  this  control  and  have  been 
locally  dominant. 


CHAPTER  X. 

MINING,  TRANSPORTATION,    PRODUCTION,   RESERVE,  OWN- 
ERSHIP, PRICES  OF  ORES,  FURNACE  USE  OF  ORES. 

METHODS  OF  ]MI?fi:ffG. 

A  monograph  on  the.Mesabi  district  would  not  be  complete  without 
brief  reference  to  the  interesting  economic  features  connected  with  its 
mining  industries.  The  writer  is  not  competent  to  make  a  technically  com- 
plete discussion  of  this  subject,  nor  would  it  be  advisable  to  give  the  neces- 
sary space  to  such  a  discussion  in  a  monograph  of  this  nature.  It  is  the 
aim  in  the  following  chapter  to  present  in  nontechnical  language  a  brief 
sketch  of  the  more  interesting  and  obvious  economic  features  of  the  district 
which  would  be  likelv  to  interest  the  general  reader  and  observer. 

MINING   BY   STEAM    SHOVELS    IN   OPEN   CUTS. 

Because  of  the  soft  character  of  the  Mesabi  ores  and  their  occurrence 
in  shallow  deposits  much  of  the  ore  niay  be  loaded  directly  by  steam 
shovels  on  railway  cars  in  open  cuts.  This  method  may  be  observed  (in 
1902)  in  the  Stevenson,  Mahoning,  Mountain  Iron,  Oliver,  Sauntiy-Alpena, 
Hale,  Leetouia,  Malta,  Spartan  (part),  Fayal  (part),  Biwabik,  and  other 
mines.  (See  Pis.  XXIV-XXVIII,  XXX-XXXIII.)  Several  more  mines 
will  use  this  method  in  the  immediate  future.  In  1902  about  47  pei-  cent 
of  the  Mesabi  ore  was  mined  by  steam  shovel. 

The  procedure  is  simple.  The  glacial  drift,  or  "overburden,"  or 
"surface,"  is  stripped  from  the  ore  with  steam  shovels.  The  thickness  of 
tlie  drift  removed  may  be  only  a  few  feet,  or  as  much  as  85  feet.  The 
average  is  between  20  and  40  feet.  Among  mining  men  the  expression  is 
common  that  it  pa}-s  to  strip  as  great  a  thickness  of  drift  as  there  is  ore 
beneath.  However,  factors  other  than  thickness  of  the  ore  beneath 
frequently  determine  the  amount  of  drift  tliat  is  advisable  to  attempt  to 
remove.     The  total   jiiiiount   of  drift  whicli    has  had    to   lie  removed  from 

•.'SO 


z<- 


-V 


ECONOMIC  FEATURES.  281 

some  of  the  large  deposits  is  very  great,  numbered  by  millions  of  cubic 
yards; 

When  the  surface  of  the  ore,  or  a  part  of  it,  is  stripped,  standard-gage 
railroad  tracks  are  built  out  on  the  ore  deposit  and  steam  shovels  make  a  cut 
through  the  ore.  In  this  first  cut  the  ore  is  either  thrown  to  one  side  or  is 
loaded  on  cars  brought  on  a  parallel  track.  After  the  first  cut  the  shovel  is 
set  over  against  tlie  bank  and  another  slice  is  taken  off'  and  loaded  on  to 
cars  run  into  the  cut  already  made.  When  by  a  series  of  slices  or  cuts  the 
bank  or  bench  or  level  is  carried  back  far  enough,  work  is  begun  as  before 
on  a  lower  level,  and  in  time  this  is  followed  by  cuts  on  third  and  fourth 
levels,  carving  the  deposit  into  a  series  of  banks  or  terraces  at  sevei'al  levels, 
against  any  or  all  of  which  steam  shovels  may  work,  giving  access  to  a  great 
variety  of  ores  and  making  possible  a  large  output  in  a  short  time.  In  the 
Mountain  Iron  mine  the  open  pit  has  been  made  in  this  manner  to  rise  in  a 
series  of  terraces  or  levels  from  the  central  part  of  the  deposit  toward  the 
edges.  (See  PI.  XXVI.)  In  the  Mahoning  mine  the  cuts  have  been  first 
made  in  a  spiral  form,  lea^dng  a  bank  in  the  middle,  so  that  subsequent 
cutting  goes  on  both  toward  the  center  and  toward  the  periphery  of  the 
deposit.     (See  PI.  XXXII.) 

While  the  ore  is  soft  it  is  usually  too  compact  to  handle  economically 
without  blasting,  and  so  a  small  amount  of  this  is  done. 

The  systems  of  trackage  vary  from  mine  to  mine.  In  the  Mountain 
Iron  mine  the  tracks  enter  the  deposit  at  one  jolace,  slightly  diverge,  and 
run  nearly  parallel  to  one  another  on  the  different  levels.  (See  fig.  A  of 
PI.  XXII.)  The  main  track  goes  tlii-ough  the  deposit  and  projects  out  the 
other  end.  The  ore,  however,  is  brought  out  where  the  tracks  enter.  As 
the  ore  layers  dip  in  general  in  southerly  directions,  the  tracks  cross  their 
strike  and  enable  any  desired  grades  of  ore  to  be  reached. 

In  the  Mahoning  mine  the  tracks  enter  at  one  place,  but,  instead  of 
running  straight  through  the  deposit,  they  wind  into  the  open  pit  in  spiral 
curves,  sending  out  tangential  curves  to  the  different  levels.  (See  fig.  C  of 
PL  XXII.)  If  in  the  Mahoning  deposit  the  tracks  had  been  put  in  according 
to  the  Mountain  Iron  system,  the  shape  of  the  deposit  would  have  required 
the  tracks  to  be  run  parallel  to  the  strike  of  the  layers,  thus  making  it  difficult 
to  get.  access  to  the  variety  of  ores  present.  By  laying  the  tracks  in  spiral 
curves  this  difficulty-  was  avoided. 


282  THE  MESABI  IRON-BEARING  DISTRICT. 

Ill  the  Biwabik  mine  the  tracks  enter  the  mine  at  one  point,  make  a 
gentle  horseshoe  bend  in  the  deposit,  and  come  out  at  another  point.  (See 
fig.  B  of  PI.  XXII.)  In  1901  one  of  the  approaches  was  closed,  and  the 
tracks  have  since  come  into  the  deposit  at  one  apjjroach. 

The  Fayal  system  is  illustrated  on  PI.  XXII,  D. 

MILLING. 

This  is  a  system  adapted  to  the  soft  character  of  the  Mesabi  ores,  but 
only  a  very  small  percentage  ot  the  Mesabi  ores  is  mined  in  this  war. 
The  method  is  used  (1902)  in  the  Norman,  Auburn,  Adams  (part),  Jordan, 
Minorca  (part).  Morrow,  Sharon,  Fayal  (partj,  and  Dulutli  mines.  About 
7  per  cent  of  the  Mesabi  production  for  1902  was  mined  by  this  system. 

The  glacial  drift  is  removed  as  in  open-cut  steam-shovel  mining;  a 
shaft  is  sunk  in  the  adjacent  wall  rock  to  the  level  of  the  bottom  of  the 
dej^osit  and  drifts  or  cross  cuts  are  run  out  through  the  ore;  uprises,  or 
chutes  without  timber,  are  sent  up  to  the  top  from  the  cross  cuts  or  drifts ; 
by  blasting  tlie  ore  is  then  loosened  at  the  surface,  and  pushed  into  the 
chutes  b}'  men  with  rods  and  shovels;  it  falls  into  cars  stationed  at  the 
bottom  of  the  chutes,  is  trammed  to  the  shaft,  and  thence  hoisted  PI.  XXX 
shows  a  view  of  the  apertures  of  the  chutes,  known  as  the  "mills,"  into 
which  the  ore  is  tumbled. 

UNDERGROUND   CAVING  AND  SLICING  SYSTEMS. 

Caving  and  slicing  systems  may  be  observed  in  the  Penobscot,  Burt, 
Sellers,  Hull,  Rust,  Pillsbury,  Clark,  Union,  Chisholm,  Spruce,  Commo- 
dore, Adams  (part),  Gleiioa,  Sparta  (part),  Elba,  Franklin,  Kanawlia, 
Roberts,  Fayal  (part),  Agnew,  Corsica,  Croxton,  Day,  Fayal  (part),  Glenn, 
Grrant,  Hawkins,  La  Belle,  Laura,  Lincoln,  Long-year,  Minorca  (part), 
Pearce,  Pettit,  Utica,  Victoria,  and  Wills.  About  4(3  per  cent  of  the  Mesabi 
production  for  1902  was  mined  by  these  methods. 

Shafts  are  sunk,  usually  in  the  ore  close  to  the  wall  rock,  and  in  st)nK' 
cases  in  tlie  wall  rock,  well  to  the  bottom  of  the  deposit,  although  in 
places  not  to  the  bottom,  because  of  the  irregularities  there  likely  to  be 
encountered.  Drifts  or  cross  cuts  are  run  out,  either  diagonallv  or  in 
rectangular  systems;  timbered  raises  are  sent  up  to  the  top  of  the  deposit, 
and  the  ore  drawn  in  from  tlie  top  by  drift  slicing.     The  liottom  of  the  di-ift 


U.S.  G^EOLOGICAX  SURVEY 


MONOGRAPH  Xim    FLXXni 


MAP  OP  THH 

ADAMS  MIKE 

EVELETH,  MINNESOTA 

Scale 


800        4oi  feet 
L£GE>JD 


Qiiaiior-eecflon  and 
sixtoenth  linua 


0     L      O      Q     "U      E      T 


M      I      N     B 


T.SeN       R.  17W. 


SPRUCE 


31     I      N     I      N     O 


ECONOMIC  FEATURES.  283 

is  planked.  As  soon  as  one  level  or  slice  has  been  removed  the  surface  or 
glacial  drift  is  allowed  to  fall  in  or  is  blasted  in,  being  kept  from  mixing 
with  the  ore  by  the  planking  at  the  bottom  of  the  first  level.  Then  another 
slice  is  taken  out  below  on  a  sublevel,  the  planldng-  and  the  surface  allowed 
to  fall  in,  as  before,  and  so  on,  always  working  down. 

In  the  rooming  or  square-set  method,  rooms,  consisting  of  three  or 
more  sets  from  20  to  40  feet  wide,  are  run  up  from  the  main  drift  to  the  top 
of  the  deposit,  the  sides  being  lagged.  Pillars  of  ore  of  about  the  same  size 
as  the  rooms  are  left.  The  glacial  drift  is  allowed  to  fall  in  from  above, 
filling  the  rooms.  The  intervening  pillars  are  then  taken  out  by  slicing 
either  froxn  the  top  or  bottom. 

Tramming  is  done  mainly  by  hand  or  with  mules,  although  of  late 
electric  haulas'e  has  bearun  to  be  introduced. 

COMPARISON   OF  METHODS   OF   MINING. 

A  discussion  of  costs  is  out  of  place  in  a  report  the  subject-matter  of 
which  is  mainly  geologic;  but  even  from  the  above  cursoiy  description 
of  mine  methods  it  must  be  apparent  that  in  general  the  open-pit  method  of 
mining  is  far  the  cheapest  of  the  methods  employed;  that  the  underground 
methods  are  the  most  expensive;  and  that  the  milling-  methods  are  interme- 
diate in  cost. 

The  cheapness  of  the  open-pit  shovel  method  as  compared  with  the 
underground  mining  is  due  to  the  large  production  possible,  to  the  fact  that 
timbering  is  not  necessary,  fewer  men  are  required,  lighting  expense  is  less, 
all  the  ore  can  be  moved  (while  in  underground  methods  perhaps  10  per 
cent  is  lost),  the  ore  can  be  better  sorted,  it  has  to  be  handled  but  once, 
tramming  cost  is  saved,  and  the  hoisting  is  by  locomotive  on  a  grade  rather 
than  through  a  shaft. 

If  open-pit  steam-shovel  mining  is  so  much  cheaper  than  the  under- 
ground methods,  the  question  is  often  asked  why  any  of  the  Mesabi  ores 
are  mined  by  any  other  method.  In  order  that  a  deposit  may  be  worked 
satisfactorily  with  the  open-pit  steam-shovel  method  it  must  have 
considerable  horizontal  extent  as  compared  with  its  length  in  order  to 
afford  proper  grades  to  the  tracks.  The  deposit  must  lie  in  a  position  to 
allow  of  an  easy  grade  to  the  approach;  this  condition  is  met  where  a 
deposit  is  on  a  side  slope.     The  thickness  of  the  drift  to  be  removed  must 


284  THE  MESABI  IRON-BEARING  DISTRICT. 

not  be  too  great,  for  otherwise  the  cost  of  stripping  will  run  up  the  total 
cost  of  mining.  There  must  be  available  gTound  with  easy  grades  on 
which  to  deposit  the  dirt  stripped  off  from  the  ore  body.  If  there  are 
capping  layers  of  poor  or  unsalable  ores  these  must  be  removed  before  the 
good  ore  can  be  mined.  Or  if  it  is  desired  to  remove  interstratified  layers 
of  good  and  poor  ore  independently  it  is  not  practicable  by  this  method. 
Finally,  the  annual  production  must  be  large.  Without  a  large  annual 
production  the  interest  on  the  preliminary  investment  for  stripping,  rolling- 
stock,  etc.,  necessary  before  a  pound  of  ore  can  be  moved  runs  the  price  of 
ore  per  ton  up  to  a  high  figure.  This  preliminary  investment  is  in  most 
cases  large. 

On  the  other  hand,  when  a  deposit  is  opened  up  by  an  underground 
method  there  is  little  preliminary  investment;  no  g'reat  mass  of  stripping 
has  to  be  removed  and  disposed  of;  no  layers  of  poor  ore  have  to  be 
removed  before  the  good  ore  can  be  reached;  the  accessibility  of  all  parts 
of  the  deposit  does  not  depend  on  grade;  the  mine  can  be  worked  all  the 
year  round;  and  finally,  the  ore  taken  out  while  the  mine  is  being  opened 
goes  to  defra}^  current  expenses  and  to  pay  interest  on  the  investment. 
Thus  it  is  that  while,  where  conditions  allow  it,  open-cut  mining  costs  less 
than  half  that  of  underground  mining,  in  many  cases  it  is  still  advisable  to 
use  underground  methods. 

The  milling  method  is  a  combination  of  the  open-cut  and  undergi-ound 
methods,  and  combines  some  of  the  advantages  and  disadvantages  of  Ixith. 
It  costs  less  than  the  underground  methods  of  slicing  and  caving,  liecause 
the  timbering  is  less  and  all  of  the  ore  is  saved,  but  it  usually  costs  more 
than  the  open- cut  steam-shovel  method  because  of  the  shafts,  drifts,  uprises, 
and  the  tramming  and  hoisting. 

The  recent  tendency  has  been  greatly  to  increase  the  use  of  the  open- 
pit  steam-shovel  method  of  mining.  More  of  the  new  mines  are  opened  in 
this  way  than  formerly,  and  several  of  the  mines  whicli  liave  in  the  past 
used  underground  methods  will  produce  their  ore  by  steam  shovel  in  the 
future.  There  is  also  appearing  a  marked  change  in  the  policy  of  conserv- 
ing ores.  In  the  past  it  has  often  been  the  practice,  because  of  market 
conditions  or  because  of  desire  for  immediate  large  profits,  to  take  out  high- 
grade  ores  finding  ready  sale  wherever  they  were  found,  without  regard  to 
the  grades  that  were  left.     Tlie  better,  and   fm-  tlie  most   part  tlie  later, 


U.    S.    GEOLOGICAL  SURVEV 


MONOGRAPH   XLIII       PL.   XXlV 


,1.      RAILWAY    CUT    IN    APPROACH    TO    OLIVER    MINE,    VIRGINIA, 
Shows  close  jointing  and  brittle  nature  of  the  iron-bearing  formation,      The  rock  is  a  slaty  phase  of  the  ferruginous  chert. 


B.      PRELIMINARY   STRIPPING   AT  OLIVER    MINE,   VIRGINIA. 


U.   S.   GEOLOGICAL  SURVEY 


MONOGRAPH  XLIII       PL.    XX\/ 


hkMiMM^^^^ 


VIEWS   OF    OLIVER    MINE,    VIRGINIA,    IN    1900. 
A,   Looking  west;    S,    Looking  eist. 


ECONOMIC  FEATURES.  285 

practice  has  been  to  determine  well  in  advance,  in  some  cases  even  before 
anv  mining  has  been  done,  the  grades  of  ore  and  their  distribution  for  the 
entire  deposit,  and  in  the  mining  to  make  such  selection  and  combination  of 
these  grades  as  to  leave  the  lo\Yest  surplus  of  undesirable  ores.  The  mining 
of  the  ore  is  planned  to  the  end,  as  in  building  a  structure,  and  is  not  influ- 
enced so  largely  as  formei'ly  by  temporary  conditions  of  market  or  man- 
agement. The  change  is  possible  lai-gely  because  of  the  new  conditions  of 
ownership  whereby  the  control  of  the  mines  is  in  the  hands  of  a  few  large 
steel  interests. 

TRAlS^SPOETATIOiy. 

The  Mesabi  iron  ores  are  transported  to  Lake  Superior  by  rail,  and 
thence  by  lake  to  terminal  lake  ports.  The  average  rail  haul  is  about  75 
miles.  Three  i-ailways  can-y  the  ore,  the  Duluth,  Missabe  and  Northern,  the 
Duluth  and  Iron  Range,  and  the  Eastern  Railway  of  Minnesota. 

The  Duluth  and  Iron  Range  Railway  carried  the  ore  from  the  follow- 
ing mines  shipping  ore  in  1902:  Auburn,  Fayal,  Elba,  Genoa,  Sparta, 
Roberts,  Hale,  Malta,  Kanawha,  Spruce,  Union,  Franklin,  Corsica,  Minorca, 
Bessemer,  Pettit,  La  Belle,  Section  33,  Victoria,  and  Wills. 

The  ore  is  delivered  to  Lake  Superior  docks  at  Two  Harbors,  Minn. 
The  docks  are  five  in  number,  with  776  pockets  and  storage  capacity  of 
162,040  tons.  The  same  railway  also  carries  the  ore  for  the  Vermilion 
iron  range,  and  the  docks  serve  for  both  ranges. 

The  Duluth,  Missabe  and  Northern  Railway  carried  the  ore  from  the 
following  mines:  Adams,  Duluth,  Pillsbury,  Sellers,  Burt,  Oliver,  Day, 
Hull,  Rust,  Biwabik,  Mountain  Iron,  Glenn,  Lincoln,  and  Spruce. 

The  ore  is  delivered  to  Lake  Superior  docks  at  West  Duluth,  Minn. 
These  docks  are  three  in  number,  with  960  pockets  and  storage  capacity 
of  167,040  tons  (PL  XXXIII,  B). 

The  Eastern  Railway  of  Minnesota  hauled  ore  from  the  following 
mines:  Chisholm,  Clark,  Commodore,  Mahoning,  Penobscot,  Sauntry, 
Stevenson,  Alpena,  Sharon,  Grant,  Pearce,  Jordan,  Longyear,  Agnew, 
Morrow,  Croxton,  Utica,  Laura,  Hawkins,  and  Leetonia. 

The  ore  is  delivered  to  Lake  Superior  docks  at  Superior,  Wis.,  two  in 
number,  with  500  pockets  and  a  storage  capacity  of  103,000  tons.  A  third 
dock  is  in  process  of  construction. 


28Q  THE  MESABI  IRON-BEARING  DISTRICT.     " 

The    tonnage   of   ore  from    the    Mesabi   range    carried   by  the  three 
railways  in  1902  is  as  follows: 

Ore  carried  from  Mesabi  district  hy  different  railways. 


Duluth  and  Iron  Kange 

Duluth,  Missabe  and  Northern. 
Eastern  Minnesota 

Total 


1902. 


3, 538, 978 
5, 610, 407 
4, 180, 568 


13, 329, 953 


In  density  of  traffic  and  freight  earnings  per  mile  of  track  there  are 
but  half  a  dozen  roads  in  the  United  States  which  compare  with  these  three 
railways,  a  fact  brought  out  in  the  famous  ore-rate  case  in  which  the  State 
of  Minnesota  has  made  an  unsuccessful  attempt  to  have  the  rate  on  ore 
reduced.  The  rate  from  the  Mesabi  range  to  Lake  Superior  is  uniformly 
80  cents  per  ton. 

The  Duluth  and  Iron  Range  and  the  Duluth,  Missabe  and  Northern 
railways  and  the  docks  at  their  termini  are  owned  by  the  United  States 
Steel  Corporation.  About  79  per  cent  of  the  ore  which  these  roads;  hauled 
in  1902  was  owned  by  the  same  corporation.  The  Eastern  Railway  of 
Minnesota,  as  already  noted,  is  a  part  of  the  Great  Northern  system.  None 
of  the  ore  earned  by  this  road  was  mined  by  the  Great  Northern,  iDut  by 
traffic  contracts  and  fees  it  controlled  70  per  cent  of  the  ore  it  carried. 

The  ore  has  in  the  past  been  largely  carried  in  wooden  cars  with  a 
capacity  of  25  to  35  tons.  Within  the  last  two  years,  however,  steel  cars 
of  50  tons  capacity  have  been  extensively  introduced. 

Revenue  loads  for  single  trains  in  1902  averaged  1,400  to  1,730  long 
tons  for  the  three  railways,  and  the  loads  for  double  headers  on  the  Duluth 
and  Iron  Range  Railway  averaged  about  2,100  tons. 

From  Lake  Superior  docks  the  ore  i  >  carried  in  vessels  to  Lake  Erie 
jjorts,  or  to  Chicago  and  Milwaukee.  The  rate  per  ton  has,  since  the 
opening  of  the  Mesabi  range,  varied  between  57  cents  and  $1.29.1.  The 
rate  for  1902  was  75  cents. 

The  larger  interests  in  the  Mesabi  district  control  their  own  lake 
steamers.  The  largest  fleet,  of  course,  is  that  of  the  United  States  Steel 
Corporation,  which  numbered  112  vessels  in  1902,  enabling  this  comjianyto 
carry  aljout  63  i^er  cent  of  its  own  oi'e  for  that  year. 


U.   S.   GEOLOGICAL  SURVEY 


MONOGRAPH  XLIII       PL.    XXVI 


VIEWS   OF    MOUNTAIN    IRON    MINE. 
A,    Looking  north  through  mine;    B,    Sieam  shovel  "  bucking  "  bank  of  ore. 


U.   S.   GEOLOGICAL  SUftVEY 


A.     SHARON    MINE,   SHOWING  STRIPPING   OPERATIONS. 


«"    - 


4^^ 


'*<^*, 


* 


'*--  M, 


a.     AUBURN    MINE   OPEN    PIT   AND   SHAFT. 


ECONOMIC  FEATURES. 


287 


PRODUCTION. 


The  following  table,  taken  (except  for  1902)  from  the  Iron  Trade 
Review,  shows  the  shipment,  in  gross  tons,  of  iron  ore  in  the  Mesabi  district 
since  its  opening  in  1892: 

Prochiction  in  Mesabi  district. 


Name  of  mine. 

1892. 

1893. 

1894. 

1895. 

1896. 

1897. 

Adams 

59, 141 

234, 562 

17,  723 

131,478 

242,  565 

,    16,261 

57, 324 
96,  280 
22,  063 

170  738 

Aetna  (Lowmore)       ' 

1,645 

Auburn 

108,  210 

90, 048 

213.  853 

376, 970 
247,  069 
359, 020 

17, 187 
47,  700 

175  263 

Biwabik 

151,500 
24,416 
26, 372 

427  464 

Canton 

Cincinnati 

32,  912 
12,  215 

Cloquet  ( Vega) 

5,628 
7,213 

65, 137 
37,  626 

60,  798 

Duluth 

Fayal 

136,  601 
286, 423 

248,  645 

231, 086 

17, 136 

70, 006 

67, 659 

167, 245 

642, 939 
30, 128 

Franklin 

46,617 

223, 399 

Genoa 

309, 514 
13  728 

Hale 

3,616 

24, 167 

31, 004 

58, 123 

117, 884 

Lake  Superior  Group        .         

259  912 

Mahoning .... 

519  892 

Minnewas 

13,  858 
119, 818 

2,140 

573, 440 

38, 999 

Mountain  Iron  ( and  Kath )      

4,245 

371, 274 
93, 392 
28,  943 

500, 377 

142, 021 
77, 523 
69, 925 

808,  291 

773,538 
101,  077 

Ohio 

47,  350 

OUver  (1) 

123, 015 

505, 955 

601, 072 

Penobscot 

11, 933 

Roberts 

18, 614 

Sellers 

47, 433 

153, 037 

Sparta .- 

66,  722 

Williams  (North  Cincinnati)  . 

3,046 

11, 249 

Total 

4,245 

613, 620 

1,793,052 

2,  781, 587 

2, 882, 079 

4, 275,  809 

Name  of  mine. 

189S.                    1899. 

1900. 

1901. 

1902. 

Totals. 

Adams 

390, 860         720, 474 

777, 346 

829, 118 

1, 242, 923 

4,  425, 162 
19,  368 

Aetna  (Lowmore ) 

Agnew 

45, 582 

38, 283 

623, 128 

45  582 

Auburn 

235, 630 

385, 992 

553,  836 

99, 498 

263.  69^ 

427,510 
410, 074 

2, 143, 028 

4,  053.  732 

713,048 

Biwabik 

Canton  

383, 

180 

924, 868 

:^88' 


THE  MESABI  IRON-BEARING  DISTRICT. 

Production  in  Mesabi  district — -Continued. 


Name  of  mine. 

isas. 

1899. 

1900. 

1901. 

1902. 

Totals. 

34,573 

200,  629 

235. 202 

Pincinnati 

246 

134 

613 

163 

15 

768 

86 

18 

127 

744 

570 

7,505 

1,370 

1,868 

23 

51 

279 

5 

147 

145 

70 

2,320 

3 

28 

87 

22 

4,783 

413 

15 

35 

35 

7,462 

421 

714 

3,143 

54 

704 

17 

666 

190 

041 

Clark                  

63,  071 

199, 566 

350,  799 

436 

1,621 

444 

Columbia                               .... 

15, 627 
35, 546 
26, 838 

697 

Commodore             .       

80,  494 

152, 947 

278, 416 

65, 833 

59, 294 

18, 594 

106, 516 

150, 220 

214, 447 

1,919,173 

84,  534 

399, 719 

23,  875 
51,  946 
54,  289 

5,892 
147,  931 

24,  830 
70,  753 

766, 312 

3,575 

28,  785 

87,  779 

22,  735 

1,  030, 143 

192, 874 

447 

Corsica 

13? 

Croxton                

594 

Dav          

18,651 
112, 155 
564 
575, 933 
200, 400 
279, 677 

1,975 

165, 435 

9;  547 

1,  072,  257 

60, 000 

276, 559 

14'> 

Duluth 

128, 587 
121,  707 
1,  252, 504 
168, 524 
253, 651 

150, 024 

224, 630 

1,  656, 973 

39,  299 

332, 022 

047 

Elba 

895 

Fayal 

0''5 

410 

Genoa  

''78 

Glenn 

875 

Grant 

946 

Hale 

18,  807 

32, 901 

30, 929 

447 

Hawkins 

S9'> 

Jordan 

931 

14, 963 

64,218 

41, 300 

sn 

La  Belle 

753 

Lake  Superior  Group 

135, 404 

154, 326 

284, 023 

594, 761 

.5''0 

Laura 

575 

Leetonia 

785 

Lincoln  

779 

Longvear  

735 

.Mahoninc; 

520,  751 

750, 341 
28, 615 

911,021 
65, 346 

765,  872 
126, 299 

149 

Malta 

134 

Miniiewas 

998 

Minorca 

35, 500 

35, 572 

1, 629, 576 

500 

Morrow . ; 

57-> 

Mountain  Iron  (and  Rath)... 
Norman  

650, 955 
110, 141 
101,607 
349, 100 

1,137,970 

1, 001, 324 

1,058,160 

321 
13'' 

Ohio 

293, 651 

172,597 
244, 876 

073 

Oliver  (1) 

5,420 

5,131 

54,  885 

209, 431 

17, 278 

238, 122 

28, 972 

249, 837 

4, 213 

■'37 

Piarce 

885 

rcnohncot 

29,  652 

85, 619 

146, 641 

221,080 

356 

I'cttit  

•'7S 

I'illsl)urv  

99,  691 

106,  487 
57,  S47 
53,  004 

101,032 
41,965 
(iS,  560 

120,  723 

42,  756 

328, 739 

0^15 

KiLcrtH 

1R4 

Samitrv-Alpena 

700, 140 

S  •(•.  -.VA 

4,213 

ECONOMIC  FEATURES. 

Production  in  Mesahi  district — Continued. 


289 


Name  oi  mine. 

1898. 

1899. 

1900. 

1901. 

1902. 

Totals. 

Sellers 

Sharon               

112,  765 

174, 867 

56,  280 

34, 918 

56,  810 

156, 426 

279, 515 

666, 273 

93, 109 

193, 428 

224,  526 

252,  674 

543, 397 

1,  424,  864 

103,  521 

9,009 

26, 465 

772,  728 
281  336 

Sparta 

Spruce            

226, 156 

237, 143 

202, 144 

101, 675 

56, 031 

8,297 

1, 141,  265 
924,  587 

Stevenson 

2, 147, 168 

Union  . 

204, 927 

9,009 

26, 465 

44,  890 

12,  159 

Utica 

Victoria 

! 

Williams  ( North  Cincinnati ) . 

12, 357 

18, 238 

Wills 

12, 159 

Total      

4, 613, 766 

6, 626, 384 

7, 809, 535 

9, 004,  890 

13, 329, 953 

53,  734, 920 

In  1891  the  district  had  uot  been  opened  up.  In  1895  the  Mesabi  dis- 
trict became  the  largest  producer  in  the  Lake  Superior  region,  that  year 
passing  the  Marquette  district  which,  since  1854,  had  held  first  place.  In 
1901  the  total  production  of  the  Mesabi  district  was  44  per  cent  of  that  of 
the  Lake  Superior  region,  and  two  and  one-half  times  as  much  as  its  near- 
est competitor,  the  Menominee,  which  in  1901  for  the  first  time  surpassed 
the  Marquette  district  in  production.  The  combined  production  of  four  of 
the  largest  producers  in  the  Mesabi  district  for  1901,  the  Mountain  Iron, 
Fayal,  Adams,  and  Mahoning-  mines,,  was  greater  than  the  total  for  the 
entire  Menominee  district  for  the  same  year.  Beginning  in  1899  the  Fayal 
and  Mountain  Iron  mines  have  shipped  over  a  million  tons  each  year,  a  rec- 
ord which  has  been  made  by  no  other  mine  in  the  world.  The  Fayal  ship- 
ment in  1901  was  1,656,973  tons,  an  amount  almost  as  great  as  the 
shipment  for  the  entire  Vermilion  range  for  the  same  year.  Largely  on 
account  of  the  Mesabi  production  the  State  of  Minnesota  in  1901  passed 
the  State  of  Michigan  as  the  larg-est  iron  producer  in  the  United  States. 
In  total  shipment  the  Mesabi  district  is  still  behind  the  Marquette  district. 
At  the  close  of  1901  the  Mesabi  district  had  shipped  a  total  of  40,404,967 
tons,  while  the  Marquette  district,  open  since  1854,  had  shipped  62,847,473 
tons. 

Comparing  the  Mesabi  shipment  for  1901  with  that  of  the  United 
States,  it  appears  that  the  Mesabi  district  shipped  33  per  cent  of  the  total. 

MON  XLIII — 03 19 


290  THE  MESABI  IRON-BEARING  DISTRICT. 

In  1902  the  Mesabi  sliipments  showed  an  increase  of  41  per  cent  over 
its  own  sliipment  for  tlie  preceding  year  and  constituted  49  per  cent  of  the 
total  Lake  Superior  shipments  for  that  year. 

An  examination  of  the  general  map  of  the  Mesabi  district  (PI.  II) 
shows  that  there  are  many  iron-ore  deposits  on  the  range  which  have  not 
yet  shipped  ore.  This  is  because  of  their  grade  or  because  of  their  late 
discovery  or  because  they  ai-e  controlled  by  companies  wliicli  have  enough 
ore  for  present  shipments  in  properties  alread}'  opened  up.  By  leaving  tlie 
ore  untouched,  taxes  and  the  interest  on  the  large  investment  necessary  to 
open  up  a  mine  are  saved.  Other  mines  liave  been  opened  up  and  but 
small  shipments  made  because  more  desirable  ores  or  more  cheaply  mined 
ores  were  controlled  by  the  same  company  in  other  mines.  With  the  mines 
already  opened  up,  including  the  steam-shovel  mines,  the  annual  production 
of  the  district  could  be  enormously  increased  without  opening  any  more 
deposits. 

RESERVE   TONNAGE. 

The  tonnage  of  individual  projjerties  ranges  from  a  few  thousand  tons 
up  to  a  possible  maximum  of  70,000,000  tons.  Several  deposits  are 
known  to  have  between  20,000,000  and  40,000,000  tons  of  ore.  The  total 
amount  of  ore  of  present  marketable  grade — that  is,  containing  above,  per- 
haps, 58  or  59  per  cent  of  metallic  iron — at  present  in  sight  on  the  Mesabi 
range  has  been  estimated  at  500,000,000  to  700,000,000  tons.  Six  hundred 
million  tons  is  a  commonly  accepted  figure.  Of  this,  with  proper  mixing, 
perhaps  60  to  70  per  cent  is  Bessemer  ore.  These  figures  are  necessarily 
based  on  incomplete  data,  but  they  are  commonly  accepted  by  those  best 
qualified  to  judge.  Ore  running  below  58  or  59  per  cent  in  metallic  iron 
is  known  to  be  present  in  enormous  quantity,  but  the  amount  lias  not  been 
estimated  nor  are  the  data  for  an  estimate  likely  to  be  available  for  some 
time  to  come.  Within  recent  years  steps  have  been  taken  to  reserve  the 
best  of  it.  Ores  running  even  as  low.  as  50  per  cent,  while  not  exploited, 
are  being  put  aside  by  the  large  companies  for  future  use.  At  the  mines 
where  it  has  been  found  necessary  to  move  low-grade  ore  in  order  to  get  at 
lilgher-grade  ores,  the  low-grade  ores  are  in  some  cases  being  stock  piled 
where  this  can  be  done  at  a  small  cost. 

The  aggregate  amount  of  high-grade  ore  in  sight  up  to  1902  on  all  the 
"old  ranges"  of  the  Lake  Superior  region   li;is  Ix'cn  thought  not  greatl}'  to 


ECONOMIC  FEATURES.  291 

exceed  350,000,000  tons.  Even  if  the  estimates  for  the  Mesabi  range  and 
the  old  ranges  are  considerably  away  from  the  truth,  it  is  certain  that  the 
Mesabi  holds  a  commanding  position  in  the  region  in  its  reserve  tonnage. 
When  it  is  remembered  that  even  before  the  discoveiy  of  the  Mesabi  ores 
the  Lake  Superior  region  was  regarded  as  the  richest  iron-ore-bearing 
region  in  the  world,  it  is  apparent  that  the  Mesabi  district  has  no  peer. 

OWNERSHIP   AND    COKTROL. 

On  the  general  map  of  the  range  are  indicated  the  principal  mining 
properties  known  up  to  the  time  the  map  was  submitted  to  the  printer. 
This  information  is  based  partly  on  a  list  and  description  of  properties 
prepared  by  Mr.  J.  H.  Gruber,  land  agent  of  the  Eastern  Railway  of 
Minnesota,  for  property  owners  in  the  Mesabi  district,  to  aid  in  the  appor- 
tionment of  taxes,  but  shows  many  subsequent  additions  and  changes. 
The  large  amount  of  exploration  and  the  rapidity  with  which  discoveries 
of  ore  have  followed  one  anotlier  make  it  certain  that  before  the  map  comes 
from  the  printers  iron-ore  properties  other  than  those  indicated  will  be 
known.  Moreover,  properties  are  rapidly  changing  hands  and  the  names 
of  the  mines  are  being  changed,  with  the  resiilt  that  some  of  the  names 
shown  on  the  map  will  be  superseded.  The  name  of  the  mine  or  lease 
rather  than  the  name  of  the  owners  is  g-iven  whenever  a  name  has  been 
assigned.  It  has  been  the  aim  to  include  only  such  parcels  of  land  as 
actually  contained  ore,  but  it  has  not  been  possible  consistently  to  follow 
this  procedure.  It  seems  best  in  some  cases  to  include  the  entire  block  of 
land  covered  by  a  well-known  lease;  for  instance,  the  Kanawha  mine  is 
shown  on  the  map  to  cover  four  forties,  while  only  the  southerly  forties 
contain  the  ore. 

For  the  sake  of  convenient  reference  by  mining  men,  the  sections 
have  been  divided  into  forties.  Where  quarter  posts  have  been  found, 
subdivision  has  been  based  on  their  location.  Where  not  found,  the  sections 
have  been  proportionately  divided.  In  this  connection  it  may  be  noted 
that  the  subdivisions  of  the  sections  in  the  vicinity  of  Virginia  and  Eveleth 
and  thence  eastward  to  Mesaba  station  have  been  made  by  Mr.  D.  L. 
Fairchild  for  the  Minnesota  Iron  Company  and  kindly  furnished  us  for  use 
on  the  accompanying  map. 


292  THE  MESABI  IRON-BEARING  DISTRICT. 

To  give  a  full  and  accurate  list  of  the  owners  of  the  iron-ore  proper- 
ties would  require  an  amount  of  labor  which  is  scarcely  warranted  by  the 
scope  of  this  report.  Moreover,  the  list  would  be  out  of  date  at  a  number 
of  points  before  the  books  come  from  the  printer,  so  I'apidly  are  the  newer 
properties  changing  hands  But  in  order  to  show  the  extent  to  which  the 
control  of  the  ore  is  concentrated  into  a  few  hands,  the  names  of  the  com- 
panies or  individuals  controlling  the  shipments  for  1902,  and  the  mines 
operated  by  them,  are  given  below : 

United  States  Steel  Corporation:  Adams,  Glenn,  Auburn,  Chisholm, 
Clark,  Day,  Duluth,  Faval,  Genoa,  Lake  Superior  group.  Mountain  Iron 
(and  Rathbone),  Oliver,  Pillsbury,  Sauntry -Alpena,  Sellers,  Spruce.  In 
1902  the  United  States  Steel  Corporation  mined  about  60  per  cent  of  the 
total  Mesabi  production. 

Republic  Iron  and  Steel  Company:  Franklin,  Union,  Victoria,  Pettit, 
Wills. 

Pickauds  Mather  Company:  Elba,  Corsica,  Minorca,  Utica,  Malta  (with 
St.  Clair). 

Interstate  Mining  Company  (Jones  &  Laughlin) :  Lincoln,  Grant. 

Donora  Mining  Company  (Union  Steel  Company) :  Penobscot,  Sharon. 

Joseph  Sellwood:  Hale,  Kanawha,  Croxton,  Leetonia,  Longyear, 
Morrow,  Pearce. 

G.  A.  St.  Clair:  Malta  (with  P.  M.  Co.),  Sparta,  Sec.  33. 

Corrigan  and  McKinney:  Stevenson,  Commodore,  Jordan. 

Todd,  Stambaugh  and  Co.:  Mahoning. 

Biwabik  Mining  Company:  Biwabik. 

Fay  Exploration  Company:  Laura. 

Drake,  Bartwell  and  Co.:  Roberts. 

Deering  Harvester  Company:  Agnew,  Hawkins. 

In  most  of  the  properties  other  parties  are  concerned,  either  in  the  fee 
or  lease,  l)ut  not  directly  in  the  operation  of  the  mine. 


#i 


I 


eT* 


:if 


ECONOMIC  FEATURES. 


293 


PRICE    OF    MESABI    ORBS    IN    C0MPARIS0:N^   AVITH    OLD    RA^STGE    ORES. 

Below  are  listed  the  prices  of  Lake  Superior  ores  at  terminal  lake 
ports  from  1891  to  1901,  quoted  from  figures  given  by  A.  I.  Findley, 
editor  of  the  Iron  Trade  Review: 


Prices  of  Lcike  Siqjerior  ores. 


Grade. 

1891.               i              1892.               1              1893. 

1894. 

Mesabi  Bessemer 

12.  25  to  %2.  65 

Mesabi  non-Bessemer 

1.85 

Marquette  specular  No.  1  Bessemer 

Marquette  specular  No.  1  non-Bessemer. 
Chapin 

S5.00 
4.25 
3.75 

4.75 
5.50 

S5.00 
4.25 
3.75 

4.50 
5.65 

S4.00 
3.65 
3.25 

4.00 
4.50 

2.90 
2.50 

Soft  hematites,  No.  1  non-Bessemer 

Gogebic,   Marquette,    and    Menominee 
No.  1  Bessemer  hematites 

2.25 
2  75 

Minnesota  No.  1  hard  Bessemer 

Vermilion  No.  1  hard  Bessemer 

3.35 

Minnesota  No.  1  hard  non-Bessemer . 

4.85 
4.85 

4. 00 
3.90 

4.65 
4.50 

3  00 

Chandler  No.  1  Bessemer 

2  95 

Marquette  extra  low-phosphorus  Besse- 

6.00 
5.50 

3  50 

Republic  and  Champion  No.  1 

5.  .50 

3  25 

Grade. 

1895. 

1896. 

1897. 

1898. 

Mesabi  Bessemer 

S2  25  to  S2  70 

§3. 25  to  §3.  75 
2.40 
4.50 
3.00 
3.65 
2.60 

4.00 
4.  55 

§2. 10  to  12. 30 

1.80  to   2.00 

2.  80  to   3.10 

2.45 

2.40 

2.25 

2. 65  to   2.85 
3.11 

?2. 15  to  §2. 25 
1  70  to    1  85 

1.90 

Marquette  specular  No.  1  Bessemer 

3  10  to   3  35 

Marquette  specular  No.  1  non-Bessemer. 
Chapin                                             

2.75 
2.55 
2.25 

2.90 
3.40 

2. 35  to   2.45 

9    5g. 

Soft  hematites,  No.  1  non-Bessemer 

Gogebic,    Marquette,   and   Menominee 

2.20 
2  75  to    2  95> 

Minnesota  No.  I  hard  Bessemer 

Vermilion  No.  1  hard  Bessemer 

3.36- 

Minnesota  No.  1  hard  non-Bessemer 

Chandler  No.  1  Bessemer              

3.00 
3.05 

3.55 
3.30 

3.25 
-     4.25 

4.90 
4.50 

2.65 
2.92J 

3.  42  to    3.46 

2.55' 
3  13; 

^larquette  extra  low-phosphorus  Besse- 
mer   

3  65' 

Republic  and  Champion  No.  1 

294 


THE  MESABI  IRON-BEARING  DISTRICT. 

Prices  of  Lake  Superior  ores — Continued. 


Grade. 


Metabi  Bessemer 

Mesabi  non-Bessemer 

Marquette  specular  No.  1  Bessemer 

Marquette  specular  No.  1  non-Bessemer. 

Chapin 

Soft  hematites,  No.  1  non-Bessemer 

Gogebic,  Marquette,  and  Menominee 
No.  1  Bessemer  hematites 

Minnesota  No.  1  hard  Bessemer 

Vermilion  No.  1  hard  Bessemer 

Minnesota  No.  1  hard  non-Bessemer 

Chandler  No.  1  Bessemer 

Marquette  extra  low-phosphorus  Besse- 
mer   • 

Republic  and  Champion  ,No.  1 


1899. 


S2.  2.5  to 
] .  90  to 
3.  21  to 


S2.40 
2.10 
3.  .50 
2.  .50 
2.  73i 
2. 00  to   2.15 

2. 80  to   3.  25 


2.65 


3.35 
3.  85  to   3. 90 


1900. 


$4.  40  to  84.  90 

4.  00  to   4.25 
5. 93  to   6.48 

5.00 

4.96 

4. 15  to    4.  25 

5.  50  to    5.  75 


1901. 


5.10 

6.00 

6.  80  to   6.90 


82.  75  to  S3. 00 
2. 35  to  2.  65 
4.  66  to  4.92 
3. 65  to  3.85 
3.78 
2.  85  to    3. 15 

4.  25  to    4.  65 


4.08 


4.62 
5.  65  to   5.  75 


FURNACE    USE    OF    MESABI    ORES. 

When  Mesabi  iron  ores  first  came  on  the  market  it  was  not  found 
practicable  to  use  them  to  a  greater  extent  than  33  per  cent  of  the  furnace 
charges,  for  the  reason  that,  because  of  their  soft  character,  they  packed  in 
the  furnace.  The  term  "flue  dust"  was  commonly  applied  to  them.  Since 
that  time  the  percentage  used  in  the  furnaces  has  steadily  increased  until  in 
1902  the  average  furnace  mixture  contained  49  per  cent  of  Mesabi  iron  ore 
and  51  per  cent  of  old  range  ores.  In  other  words,  they  are  mixed  on  an 
average  in  about  the  proportion  of  their  production.  Individual  ores  are 
used  in  proportions  ranging  from  35  to  100  per  cent  of  the  ore  burden.  It 
is  not  unreasonable  to  expect  that,  considering  the  relative  abundance  of 
the  Mesabi  and  the  old  range  ores  available,  the  percentage  of  i\lesal)i  ores 
used  in  furnace  charges  will  slowly  increase  in  the  future  as  it  has  in  the 
past.  Tliis  change  results  in  part  from  the  present  tendency  to  decrease 
tlie  heiffilt  of  furnaces  from  tlie  maximuni  sizes  reached  in  late  A^ears,  thus 
giving  the  ore  less  opportunity  t(  >  pack. 


U.   S.   GEOLOGICAL   SURVEY 


MONOGRAPH    XLIII       PL.    XXX 


VIEWS    OF    FAYAL    MINE. 
A.    Mills;    B,    Milling  with  steam  shovel. 


U.   S.   GEOLOGICAL    SURVEY 


MONOGRAPH    XLtll       PL.    XXXI 


A.      SAUNTRY    MINE,    LOOKING    NORTH. 

The  layers  of  ore  in  both  banks  can  be  seen  dipping  to  the  west.  At  the  north  end  of  the  cut  the  ore  has  been  cut  back  to  the  rock  fferru- 
ginous  chert)  and  it  there  appears  that  the  rock  layers  dip  westward  at  the  same  angle.  While  the  layers  of  ore  and  rock  may  be  gently 
flexed  into  a  great  syncline,  pitching  to  the  west,  this  flexure  would  be  likely  to  be  overlooked  because  of  the  great  extent  of  the  oie 
deposit  along  the  strike  of  the  monoclinally  tilted  strata. 


B.      FAYAL   MINE,   SHOWING  STEAM    SHOVEL   "BUCKING"    BANK  OF  ORE. 


CHAPTER  XL 

EXPLORATION. 

Exploration  for  iron  ore  in  the  Mesabi  district  is  partly  a  matter  of 
chance,  as  it  lunst  be  in  any  ore-bearing  district  and  especially  in  a 
district  heavily  covered  with  glacial  drift.  The  iron  ore  thns  far  found  in 
the  Mesabi  district,  however,  has  been  within  certain  limits. 

Ore  is  confined  to  the  Biwabik  formation  of  the  Upper  Huronian 
series.  The  accompanying  g-eologic  map  shows  the  distiibution  of  the 
iron  formation  as  indicated  by  the  facts  available  n]3  to  the  time  of  its  pub- 
lication. Where  exploration  has  been  insufficient,  as  ill  the  western  portion 
of  the  district,  further  work  will  show  chang-es  in  the  iron-formation 
boundaries.  Examination  of  the  drift  may  help  to  determine  the  boundaries 
m  doubtful  areas,  for  it  is  an  observed  fact  that  fragments  of  the  iron 
formation  have  not  been  earned  far  in  great  abundance.  The  drift  frag- 
ments have  been  carried  in  one  direction,  from  northeast  to  southwest,  and 
the  discovery  of  fragments  of  iron -formation  rocks  indicates  that  the  iron 
formation  must  be  either  beneath  or  to  the  northeast.  Magnetic  work  may 
also  be  of  assistance  in  locating  the  iron  formation.  In  the  productive 
portion  of  the  district  the  iron  formation  is  essentially  nonmagnetic,  yet 
over  the  area  of  the  formation  the  magnetic  attraction  everywhere  shows 
minor  disturbance  and  this  disturbance  is  particularly  marked  near  the 
northern  boundary  of  the  iron  formation. 

The  workable  deposits  thus  far  discovered  are  confined  to  the  portion 
of  the  iron  formation  west  of  Mailman  camp,  in  range  14.  East  of  Mesaba 
station  the  oxide  is  largely  magnetite  and  the  associated  rocks  are  actinolitic, 
griineritic,  slaty,  and  crystalline.  Magnetite  with  such  associates  has  not 
been  found  in  workable  deposits  eithei-  in  the  Mesabi  district  or  in  the 
Penokee-Grogebic  district,  and  if  the  explanation  of  the  origin  of  magnetites 
given  on  preceding  pages  is  correct,  there  is  reason  to  believe  that  no  large 
deposits  of  ore  will  be  found  in  this  area. 

295 


296  THE  MESABI  IRON-BEARING  DISTRICT. 

The  westernmost  ore  deposit  thus  far  discovered  is  in  R.  25  W.  The 
ores  near  this  western  limit  contain  abundant  particles  of  silica  resulting' 
from  the  disintegration  of  the  chert  associated  with  the  ore,  giving  the  ores' 
what  is  known  locally  as  a  "sandy"  character.  Such  ores  can  be  used, 
however,  bv  washing.  The  western  limit  of  the  area  in  which  ores  may 
be  found  in  the  future  is  as  yet  quite  unknown.  The  map  of  the  district 
accompanying  this  report  covers  an  area  extending  only  a  little  way  west 
of  Grand  Rapids.  It  is  certain  that  the  iron  formation  extends  well 
westward  although  deeply  covered  by  glacial  drift.  Magnetic  work  and 
examination  of  glacial  fragments  has  already  shown  the  extension  of  the 
iron-bearing  formation  for  several  miles  west  of  the  limits  of  the  map. 
Fig.  8,  p.  203,  shows  the  possible  westward  continuation  of  the  belt  and  its 
connection  with  the  Penokee-Gogebic  series.  If  the  concentration  of  the  ore 
deposits  is  dependent  upon  vigoroiis  circulation  of  the  underground  M'ater, 
it  ma}'  be  suggested  that  the  apparent  flattening  out  of  the  Giants  range 
toward  the  west  may  not  give  the  waters  of  this  area  a  sufficient  head  to 
circulate  vigorously  through  the  iron  formation  and  concentrate  the  ores. 
The  low-grade  and  sand}'  nature  of  the  ores  also  may  be  in  some  way 
connected  with  this  feature.  However,  it  may  be  that  the  rock  surface 
still  has  a  considerable  slope  which  has  been  covered  and  obscured  by  the 
drift,  and  if  this  is  the  case  there  is  no  apparent  reason  wh}-  ore  should  not 
be  there  developed.  Cej'tainly  exploration  for  ore  is  warranted  well  to  the 
west  of  Grand  Rapids. 

Unexplored  land  in  the  vicinity  of  known  ore  deposits  is  more  likely 
to  contain  ore  than  is  unexjylored  land  in  the  vicinity  of  areas  which  have 
been  explored  and  found  barren,  for  in  the  former  case  it  is  certain  that 
the  conditions  in  the  area  as  a  whole  are  favorable  to  ore  concentration, 
while  in  the  latter  they  may  or  may  not  be.  Applying  this  principle  to  the 
district  in  general,  unexplored  land  in  the  central  portion  of  the  district  is 
likely  to  yield  larger  returns  in  exploration  than  unexplored  lands  in  the 
western  and  eastern  ends  of  the  district.  While  ore  has  been  found  in  the 
western  portion  of  the  district  it  is  not  nearly  so  abundant  as  that  found 
with  an  equivalent  amount  of  exploration  in  the  central  .portion  of  the 
district.  It  has  been  estimated  that  about  5  per  cent  of  the  area  of  the 
iron  for]nation  in  the  entire  district  is  underlain  b}?-  iron  ore,  while  for  the 
central  area,  between  Mesaba  station  nml  tlie  Hawkins  mint',  in  R.  "22  W., 
iron  ore  underlies  more  thnn  8  per  cent  ot'tlie  surface. 


U,    S.    GEOLOGICAL    SUHVEV 


PANORAMIC   VIEW   OF   MAHONING    MINE. 


0<-C"w'*:-      -   -*»*■« 


PANORAMIC   VIEW   OF    BIWABIK    MINE. 


EXPLORATION.  297 

The  ore  deposits  have  not  been  found  in  the  portion  of  the  iron  forma- 
tion which  runs  under  the  Virginia  slate,  nor  are  they  Hkely  to  be  found 
there,  for  the  reasons  that  the  circulation  accomplishing  the  secondarv 
concentra,tion  probably  is  not  vigorous  undei-  the  edge  of  the  slate  (see 
pp.  266-267);  that  the  iron-formation  rock  there  f^und  has  usuall}^  a  fresh 
unaltered  green  character;  and  finally,  while  the  slate  has  been  pierced  in 
a  few  places  by  drill  holes,  no  ore  has  been  found  any  distance  under  the 
true  black  slate,  although  the  same  number  of  holes  similarly  distributed 
almost  anywhere  else  in  the  iron  formation  has  scarcely  failed  to  reveal  ore. 
Attention  is  called  to  the  fact,  however,  that  the  absence  of  ore  under  the 
black  slate  has  not  yet  been  demonstrated  by  actual  drilling.  Considering 
the  magnitude  of  the  new  fiield  opened  further  drilling  seems  wai-ranted. 

Being  confined  to  the  part  of  the  iron  formation  not  covered  by  the 
Virginia  slate,  the  ore  bodies  for  the  most  part  lie  between  elevations  1,450 
and  1,750  feet  above  sea  level.  Most  of  the  iron  formation  north  of  the 
slate  is  between  these  elevations,  so  that  this  is  simply  another  manner  of 
stating-  that  the  iron  ore  is  in  the  part  of  the  iron  formation  not  covered  by 
the  slate. 

The  iron  ore  is  more  abundant  about  midway  between  the  north  and 
south  areal  boundaries  of  the  iron  formation  than  elsewhere,  as  shown  by 
the  location  of  the  existing  deposits;  yet  many  deposits  are  known  to 
crowd  both  the  north  and  south  boundaries  of  the  iron  foi-mation. 

The  ore  deposits  ft'equently  underlie  surface  depressions.  Their  sec- 
ondary concentration  has  been  largely  though  not  entirely  along  structural 
synclines;  the  leaching  out  of  silica  during  concentration  has  caused  a 
decided  slump  in  the  deposits;  subaerial  and  glacial  erosion  has  cut  down 
the  deposits  perhaj^s  to  a  larger  extent  than  the  surrounding  harder  rocks. 
The  heavy  and  irregular  mantle  of  drift  deposited  over  the  area  by  the 
glaciers  has  tended  to  obscure  depressions,  but  in  a  large  number  of  cases 
the  underlying  rock  trough  receives  some  expression  in  the  overlying  drift. 
Surface-drainage  lines  therefore  are  excellent  areas  to  prospect.  This  does 
not  mean  that  all  surface-drainage  lines  mark  the  course  of  ore  deposits, 
for  the  ores  have  not  developed  in  all  rock  synclines,  and,  moreover,  the 
depression  in  the  glacial  drift  containing  the  surface  drainage  may  be  quite 
inde^jendent  of  the  underlying  i-ock  topogTaphy.  Neither  is  it  true  that 
every  ore  deposit  occurs  in  a  depression  in  the  rock  surface,  for,  as  shown 


298  THE  MESABI  IRON-BEARING  DISTRICT. 

on  pages  269-274,  the  ores  iu  developiug  along  planes  of  weakness  below 
water  level  have  not  necessarily  been  confined  to  rock  troughs,  and  often 
where  so  confined  subsequent  erosion  may  have  cut  down  adjacent  areas  so 
irreo-ularly  as  to  leave  the  ore  in  higher  areas  than  the  surrounding  rocks. 

Northward  swings  of  the  northern  margin  of  the  iron  formation  or  of 
the  Pokegama  quartzite  may  mark  synclinal  basins  formed  by  tlie  folding 
of  the  Upper  Huronian  series  (though  they  may  also  mark  areas  in  which 
erosion  has  not  cut  down  so  deep  as  in  adjacent  areas).  As  the  ores 
commonly  develop  in  synclines,  the  location  of  the  synclines  in  this  manner 
may  be  of  assistance.  As  a  matter  of  fact,  however,  the  position  of  the 
northern  margin  of  the  iron  formation  is  for  the  most  part  known  after,  and 
as  a  result  of,  exploration  in  the  iron  formation. 

The  ore  deposits  being  localized  by  underground  circulation  and  the 
underground  circulation  being  limited  by  slat}'  layers,  the  location  of  such 
layers  may  give  some  information  as  to  the  direction  in  which  the  explora- 
tion should  be  carried.  Because  of  the  dip  such  slaty  rocks  may  come  to 
the  rock  surface.  It  is  a  fact  that  there  is  scarcely  a  large  deposit  in  the 
district  which  does  not  show  layers  of  paint  rock,  the  altered  equivalent  of 
slate,  below  or  above,  dividing  the  deposit,  or  at  any  or  all  horizons.  But 
in  a  given  locality  there  is  difficulty  in  determining  whether  the  slate  is 
really  an  interstratified  layer  in  the  iron  formation,  which  may  be  associ- 
ated with  ore,  or  is  the  Virginia  slate,  which  is  probabl}-  not  associated 
with  ore. 

In  general  the  slates  within  the  iron  formation  are  perhaps  more 
jointed  and  broken  up  into  small  parallelopiped  blocks  than  is  the  Virginia 
slate;  thej^  have  a  predominance  of  red  and  brown  tones,  due  to  their 
large  content  of  iron,  as  contrasted  to  gray  and  black  tones  in  the  Virginia 
slate;  they  are  more  siliceous  and  brittle;  and  they  contain  a  lower  percent- 
age of  alumina.  Because  discrimination  by  these  criteria  is  so  frequently 
doubtful,  the  position  of  the  slate  with  reference  to  the  supposed  iron- 
formation  boundary,  or  with  reference  to  surrounding  explorations  showing- 
iron  formation  or  slate,  is  likely  to  be  the  guiding  criterion  in  determining 
whether  the  slate  belongs  to  the  iron  formation. or  to  the  Virginia  slate. 

Tlie  ore  deposits  have  not  yet  been  found  to  be  covered  by  any  con- 
siderable thickness  of  barren  rock  for  an}-  large  proportion  of  their  area, 
although  shelves  and  irregular  masses  of  rock  project  from  the  walls  out 


Ij.    S.   GEOLOGICAL    SURVEY 


Monograph  xliii     pl.  xxxili 


A.     VIEW   OF    HALE    MINE,   SHOWING    MONOCLINAL   DIP   OF   STRATA   OF   ORE   AND    ROCK. 

The  steeply  dipping  strata  on  the  south  are  ferruginous  chert.  The  longer  dimensions  of  the  ore  deposit  are  parallel  to  the  ferruginous  chert  wall. 
The  deposit  extends  for  a  considerable  distance  to  the  west;  its  continuation  is  mined  at  the  Kanawha  shaft,  to  be  seen  in  the  distance. 
The  layers  of  ore  are  continuous  with  those  of  the  wall  rock  to  the  south,  although  showing  minor  disturbances  at  the  contact.  It  is  apparent 
from  this  view  that  the  ore  is  not  in  a  pitching  trough  formed  by  the  folding  of  the  iron-formation  layers,  but  is  really  in  a  long,  narrow  basin, 
upon  the  upper  edges  of  monoclinal  tilted  strata. 


B.      DULUTH,    MISSABE   AND    NORTHERN    ORE    DOCKS    AT    DULUTH. 


EXPLORATION.  299 

over  the  deposit  or  into  the  deposit,  or  islands  of  rock  may  be  surrounded 
above,  below,  and  on  the  sides  by  ore.  Wherever  drills  have  reached  ore 
in  quantity  after  penetrating  any  considerable  thickness  of  rock  it  has  been 
found  that  the  ore  appears  at  the  surface  a  short  distance  away.  Up  to  the 
present  time  thei'e  has  not  been  enough  deep  exploration  in  rock  to  pi'ove 
the  nonexistence  of  ore  in  bodies  entirely  covered  b}^  rock  and  nowhere 
reaching  to  the  surface.  In  view  of  the  fact  that  all  ore  thus  far  known  is 
only  very  locally  and  very  partially  covered  by  rock,  shallow  exploration 
over  wide  areas  is  likely  to  show  the  greatest  percentage  of  finds,  but  deep 
drilling  is  also  warranted.  In  the  future  many  areas  in  which  there  has 
been  shallow  exploration  will  need  to  be  explored  deeply  to  prove  whether 
or  not  ore  actually  occurs  beneath  them. 

The  ore  deposits  are  associated  with  characteristic  altered  varieties  of 
the  iron  formation  which  are  familiar  to  all  who  have  worked  long  with 
the  ores.  Nearly  every  mining  man  or  explorer  has  in  mind  certain  phases 
of  the  ferruginous  chert  which  he  is  accustomed  to  associate  with  ore 
deposits.  Pis.  X  and  XI  show  several  of  the  phases  frequently  associated 
with  ore.  On  the  other  hand,  other  phases  of  the  iron  formation  are  seldom 
found  in  association  with  ore,  and  thus  are  avoided  in  exploration.  Fig's.  A 
and  B,  Pis.  VIII  and  XII,  represent  certain  of  these  phases.  Further  dis- 
criminations are  made  by  Mesabi  explorers,  but  they  are  not  described 
because  of  doubt  as  to  their  general  application. 

The  question  has  been  asked,  "What  are  the  chances  of  finding  ore  in 
the  Lower  Huronian  and  Archean  rocks!"  Iron  ore  is  known  in  both  series 
in  other  districts  of  the  Lake  Superior  region,  and  there  is  no  a  priori  reason 
why  ore  should  not  be  found  in  either  or  both  in  the  Mesabi  district.  How- 
ever, both  series  are  fairly  well  exposed,  have  been  thoroughly  examined, 
and  not  only  has  no  iron  ore  been  foiind,  beyond  a  few  ferruginous  dis- 
colorations  in  the  hornblende-schists,  but  no  iron-formation  rocks  have  been 
discovered.  Stray  fragments  in  the  conglomerate  at  the  base  of  the  Upper 
Huronian  show  that  iron-formation  rocks  were  present  in  the  Lower 
Huronian  and  Archean  areas  from  which  the  fragments  of  the  conglomerate 
were  derived,  but  they  may  have  come  from  a  considerable  distance  outside 
the  Mesabi  district,  perhaps  from  the  Vermilion. 

Exploration  should  be  governed  by  the  above  conditions  so  far  as  they 
are  known,  but  it  is  never  possible  to  know  all  of  these   conditions  in 


300  THE  MESABI  IRON-BEARING  DISTRICT. 

advance  of  exploration.  When  the  district  was  first  explored  none  of  them 
were  known.  To-day  but  a  part  of  them  are  available  for  locating  explora- 
tion, but  all  of  them  niay  be  of  assistance  in  interpreting  the  facts  brought 
to  light  after  exploration  has  once  begun.  As  a  matter  of  practice  but  little 
exploration  has  been  done  in  Avhich  all  these  criteria  have  been  taken 
advantage  of,  particularly  the  criteria  developed  from  the  flowage  of  ground 
water.  In  most  of  the  cases  a  part  of  them  have  been  used,  but  they 
have  been  subordinate  to  other  conditions — the  lands  available  for  explora- 
tion at  the  time,  favorable  terms,  etc.  The  unit  of  land  transfers  is  one- 
sixteenth  of  a  section,  or  40  acres,  and  it  has  been  a  very  common  practice 
to  put  the  holes  down  systematically  over  the  "forty"  regardless  of  any 
criteria  for  closer  location  of  the  ore  which  might  have  been  present. 

Exploration  is  done  in  the  Mesabi  district  b^s'  test  pitting  with  pick  and 
shovel,  by  churn  drilling-,  and  b}"  diamond  drilling-.  In  the  early  days  the 
work  was  done  almost  entirely  by  test  pitting-.  In  later  years  drilling  has 
come  commonly  into  use.  In  the  year  1902,  200  drills,  diamond  drills 
and  churn  drills,  were  continuously  in  use  in  the  Mesabi  district.  A  hole 
is  put  down  entirely  by  drilling  or  by  test  pitting  until  water  or  rock  is 
struck  and  then  by  drilling  below.  Because  of  the  fairly  soft  character  of 
the  formations  churn  drilling,  in  which  the  cutting  is  done  by  percussion, 
is  more  common  than  diamond  di'illing,  in  which  the  cutting  is  done  by 
rotation  of  a  steel  bit  with  diamonds  set  in  its  periphery.  The  cost  of  test 
pitting  since  the  opening  of  the  range  has  varied  from  Si. 25  to  $3  per 
foot;  $1.25  is  the  present  price.  The  cost  of  drilling  has  ranged  from 
S2  to  $3.50  per  foot  for  ore  and  from  $4  to  $7  for  rock,  depending  upon 
the  nature  of  the  ore  or  rock.     The  higher  prices  are  the  later  ones. 

E.  J.  Long-year  has  called  attention  to  errors  in  determining  the  true 
composition  of  an  ore  by  drilling.  The  choppings  of  the  drill  are  brought 
to  the  surface  by  water  forced  through  casing  pipes,  are  allowed  to  settle, 
are  di-ied,  and  then  analyzed.  If  the  ore  is  not  allowed  to  settle  for  a 
considerable  length  of  time  the  lighter  materials  associated  with  the  ore  are 
likely  to  be  retained  in  suspension,  and  the  analyses  of  the  ore  therefore 
show  a  higher  percentage  of  ore  and  phos])horus  than  they  ought  to.  A 
mixture  of  blue  ore,  brown  ore,  yellow  ore,  and  paint  rock  was  analyzed, 
and  found  to  contain  59.23  percent  of  iron  and  .087  per  cent  of  phosphorus. 
When  treated  with  water  and  allowed  to  settle  for  twenty  minutes,  dried, 


EXPLORATION.  301 

aud  analyzed,  the  content  of  iron  was  60.80  per  cent  and  the  content  of 
phosphorus  .094  per  cent.  When  allowed  to  settle  for  sixteen  hours,  dried, 
and  analyzed,  the  content  of  iron  was  found  to  be  59.67  per  cent  and  that 
of  phosphorus  .088  per  cent." 

In  no  other  district  of  the  Lake  Superior  region,  or  for  that  matter  of 
any  other  region,  have  the  rewards  for  exploration  been  so  great  as  in  the 
Mesabi.  Since  the  opening  of  the  range  practically  every  explorer  who 
has  gone  at  the  -^ork  systematically  and  on  a  large  enough  scale  has  suc- 
ceeded in  finding  ore. 


a  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  XXVII,  1898,  p.  540. 


INDEX 


Actinolite-magnetite-schists  from  Birch  Lake,  43-15. 

from  Penokee-Gogebic  district,  43,  44. 
Actinolite.    {See  Ampbiboles. ) 
Adams  mine,  analyses  of  iron  ores  from,  214. 

contour  map  of,  282. 

depth  of,  208. 

ferruginous  chert,  analyses  of,  139-140. 

milling  in,  282. 

quartzite  in  Biwabik  formation,  154. 

shipment  from,  287,  289. 

slate  in  Biwabik  formation,  analyses  of,  144-145. 

transportation  of  iron  ores,  285. 

underground  mining  in,  282. 

view  of,  292,  294. 
Adularia  in  iron-ore  deposits,  211. 
.(Etna  mine,  shipment  from,  287. 

transportation  of  iron  ores,  285. 
Agnew  mine,  shipment  from,  287. 

transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Agriculture  of  Mesabi  district,  22. 
Aitken,  Minn.,  quartzite  near,  203 
Akeley  Lake,  analysis  of  magnetite  from.  221. 

Animikie  of,  45.  52,  54,  159,  201. 

literature  on,  61. 

relations  of  gabbro  to  adjacent  formations,  183. 
Albite  in  graywacke  of  Penokee-Gogebic  district,  174. 

in  spilosite  in  Crystal  Falls  district,  172, 174. 
Algonkian  system  of  Lake  Superior  region,  200. 
Allen,  E.  T.,  analyses  by,  139-140,  144-145,  156,  264. 
Allen.  James,  referred  to,  25.  , 

summary  of  literature,  31. 
Allen  Junction,  gabbro  near,  36,  182-183. 
Alpena  mine,  transportation  of  iron  ores,  285. 
Alumina  in  glauconite,  242. 

in  greenalite  rocks,  242. 

h;i  iron-ore  deposits,  219. 

in  paint  rock.  223. 
American  Iron  and  Steel  Association,  referred  to,  212,  213. 
Ampbiboles,  alteration  from  greenalite,  102,  107,  117,  237; 
plate  of.  106. 

in  ferruginous  chert,  119,  138,141-143,  159.  273;  photomi- 
crograph of,  136. 

in  iron-ore  deposits,  211. 

in  quartzite,  92. 

in  slates.  143, 144. 
Amphibolite  of  Archean,  described,  67. 
Amphibolitic  chert,  analyses  of,  141. 
Amygdaloidal  texture  in  basalt,  65. 
Analyses  of  amphibolitic  cherts,  141. 

of  cordierite  hornstone,  172. 

of  ferruginous  cherts,  140. 


Analyses  of  ferric  hydrate,  262. 

of  glauconite,  47,  240. 

of  greenalite  rocks.  108,  245-246. 

of  iron  ores,  214-217,  221-223,  262. 

of  paint  rock,  149-150. 

of  phosphorus  in  hard  and  soft  iron  ores,  220. 

of  quartzites  of  Biwabik  formation,  156. 

of  samples  in  drilling,  300-301. 

of  silica  powder,  210. 

of  siliceous  slate  of  Birch  Lake,  153. 

of  slate  in  Biwabik  formation,  144. 

of  Virginia  slate.  170. 

of  water  from  iron  formation,  264, 
Andalusite  in  Biwabik  formation  near  contact  with  Embar- 
rass granite,  160,  163. 

of  Embarrass  granite,  187. 
Animikie,  use  of  term,  27,  202. 
Animikie  series,  41-i2,  51,  55. 

correlation  of.  30,  34,  36,  41,  50,  52,  53,  56-57,  201. 

distribution  of,  40,  45,  52,  55,  56. 

iron  ores,  origin  of.  37. 

relations  to  other  series,  36,  39,  43,  45,  46,  49,  56,  57,  58. 

(See  Upper  Huroniau.) 
Anorthosite  of  Beaver  Bay  and  Duluth  gabbro,  58, 183. 
Apatite  in  altered  Lower  Huronian  rocks,  83. 

in  iron-ore  deposits,  211. 

in  Michigan  iron  ranges,  275. 
Aplite  dikes  in  Lower  Huronian  granite,  79. 
Archean,  use  of  term,  202. 

Archean  area,  Lower  Huronian  in,  63,  67,  69-70. 
Archean  series,  described,  63-71;  summary,  13. 

adjacent  to  iron-ore  deposits,  227,  230. 

distribution  of,  24,  60,  63,  66,  70. 

erosion  of,  195. 

granite  of,  origin  of,  58,  59. 

greenstone  altering  to  gabbro,  59. 

greenstone  as  a  source  of  iron  for  iron  formation,  255. 

greenstone  near  Kimberly.  Minn.,  203. 

inclusions  in  Lower  Huronian  granite,  80. 

of  Lake  Superior  region,  200. 

of  Mississippi  River,  204. 

of  northern  Minnesota,  52 

pebbles  in  Lower  Huronian  conglomerate,  70,  76. 

possibility  of  iron  ores  in,  299. 

relations  to  other  series  23,  52,  70,  71,  86,  195;  described 
70-71. 

sediments  in  Marquette  and  Vermilion  districts,  77. 

similarity  to  Crystal  Ealls  and  Vermilion  rocks,  64. 
Arcturusmine,  conglomerate  of  Biwabik  formation,  154. 

Cretaceous  near,  190. 

quartzite  at,  90. 
Attraction,  magnetic.     (See  Magnetic  attraction). 

303 


304 


INDEX. 


Auburn  mine,  analyses  of  iron  ores  from,  214. 

depth  of.  20S 

milling  in,  282. 

shipment  from.  287. 

texture  of  iron  ore  in.  224. 

transportation  of  iron  ores.  285. 

view  of  open  pit  and  shaft,  286.  ■ 
Augrite  in  contact  rooks  of  gabbro,  45.  59,  160. 

of  Dululh  gabbro,  183. 
Augite-granite  near  Mallmnn  camp,  78  (specimen  45435). 
Bacon,  D.  S.,  referred  to,  61. 
Bacteria,  iron.    {SeelTon  bacteria.) 
Bailey,  C.  E.,  referred  to,  61. 
Bain,  H.  F.. referred  to,  258. 
Bamberger,  analysis  by,  241. 
Baptism  River,  Manitou  rocks  of,  58. 

Puckwunge  conglomerate  on,  57. 
Baraboo  quartzite,  correlation  of.  41,  50,  57. 
Barron  County  quartzite.  assignment  to  Potsdam,  57. 
Basalts  of  Archean.  63;  described,  64. 

adjacent  to  iron  ore  deposits,  227. 

include  A  in  granite,  SO. 
Basement  complex  of  Lake  Superior  region,  200. 

{.Sfc  Archean.) 
Bas-swood  Lake,  granite  of,  intrusion  in  Lower  Keewatin,  55 

Lower  Keewatin  greenstone  of,  53. 
Bavalite  compared  with  greenalite  granules,  248. 
Bayley.  W.  S..  referred  to,  11,  159.  183,  222. 

summary  of  literature,  43,  49. 
Beaver  Bay  anorthosite,  58. 
Bebb.  E.  C,  indebtedness  to,  19,  20. 
Bel),  Robert,  referred  to.  34. 

Berthierine  compared  with  greenalite  granules,  248. 
Bessemer  mine,  transportation  of  iron  ores,  285. 
Bessemer  ore,  proportion  in  Mesabi  district,  219,'  290. 
Biotite  in  ferruginous  chert  near  contact  with  Embarrass 
granite,  163. 

of  Duluth  gabbro,  183. 
Birch  Lake,  Animikie  series  east  of,  45,  52. 

rocks  of,  38,  45,  52,  54.  56.  59,  78.  150-153,  182,  183,  184,  18S. 
Biwabik  formation,  46,  55;  described,  100-167;  summary,  14. 

alteration  by  granite  and  gabbro,  49,  273;  described, 
159-164:  plate  of,  152  (specimen  45138):  summary, 
17-18. 

altitude  of,  100. 

area  of,  100. 

burial  under  Virginia  state,  260. 

circulation  of  water  in,  265-268, 

conglomerates  and  quartzites  of,  described,  154-159. 

connection  with  Penokee-Gogebic  series,  296. 

contact  with  Embarrass  granite.  161;  figures  of,  162; 
plate  of,  152. 

correlation  of,  34,  37,  38,  40,  61-62:  described.  201-205, 
276,  296. 

deposition  of,  196. 

distribution  of,  100. 

erosion  of.  260,  272,  273. 

ferruginous  cherts  of.    (See  Ferruginous  cherts.) 

iron  ores  of.    {See  Iron  ore  deposits.) 

magnetic  attraction  in,  1G4-165. 

of  eastern  Mesabi,  21.  52, 136. 141, 160-163. 184,  207.  272-276. 

of  Kimburly.  204. 

paint  rock  of.  described,  149-150. 

proportion  altered  to  iron  ore,  20G,  296. 

quartzite  of,  227;  described,  154-159  (specimens  40851, 
40855,  45G55,  45665,  45668,  45687,  45753,  46020.  46021, 
46026,  46031,  46034). 

analyses  of,  156. 

in  contact  with  ferruginous  chert  of.  156;  plate  of,  126, 


Biwabik  formation,  quartzite  of.  in  contact  with  Pokegama 
quartzite,  156-157. 

relations  of.  35,  39.  46,  47.  51.  167-168.  182.  196.  230.  260. 

sideritic  and  calcareous  rocks,  described,  150-153. 

slates,  described,  145-148. 

comparisons  and  relations  with  Virginia  slate,  172-176. 

structure  of,  described,  165-166. 

thickness  of,  46,  166-167.  ISO. 
Biwabik  mine,  43.  44,  62. 

alteration  of  slate  to  paint  rock.  149,  169.  234. 

analyses  of  iron  ores  from,  214,  217. 

depth  of.  208. 

dips  in,  179.  226. 

discoverj'  of  iron  ore  in,  28. 

faulting  in,  166.  230. 

drainage  of.  235. 

ferruginous  chert,  analyses  of,  139-140. 

goethitein.  218. 

limonite  in,  210. 

manganese  in  hard  and  soft  iron  ores.  221. 

phosphorus  in  hard  and  soft  iron  ores  of,  220. 

plan  of  tracks,  280,  282. 

relations  of  iron  formation  and  slate,  175. 

shape  determined  by  circulation  of  water,  270. 

shipment  from,  2S7. 

solution  of  phosphorus  in,  275. 

steam  shovel  mining  in.  280. 

structural  relations  of  iron  ore.  228. 

transportation  of  iron  ores,  285. 

view  of,  296. 

views  of  contact  of  iron  ore  with  wall  rock.  232. 

wall  rocks  of,  227. 
Biwabik  (town) .  Archean  and  Lower  Huronian  rocks  near, 
63.  65,  68,  72. 

dips  at,  175. 

railway  connections,  23.  28. 
Black  River  Falls  rocks,  correlation  of.  37,  S8. 
Black  Hills  quartzites,  assignment  to  Taconic,  38. 
Bowlder  clay  of  Pleistocene.  192. 
Bowstring  River.  Keewatin  sediments  of,  53. 
Brackenbury.  Cyril,  referred  to.  61. 
Breccia  in  Biwabik  formation.  48,  116,  120,  137,  167,  230,  233. 

in  Lower  Huronian,  S6,  87,  96. 
Brecciation  of  iron  ore  deposits.  230,  233. 
Brooks,  referred  to,  277. 
Buckley,  E.  R..  indebtedness  to.  20. 
Buhl  (town),  railway  connections,  23. 

slate  in  Biwabik  formation,  analyses  of.  144-145. 
Burnside  Lake,  intrusion  of  grauite  in  Lower  Keewatin,  55. 
Burt  mine,  depth  of,  208. 

transportation  of  iron  ores,  285. 

underground  mining  in.  282. 
Cabotian  age  of  iron  ore,  56. 

of  Keweenawan  described,  57. 
Calamine  in  Mi.';souri.  259. 
Calcareous    rocks  of    Biwabik    formation,   101;    described 

150-153. 
Calcite  in  Clinton  iron  ores,  250,  251. 

(.SfC  Carbonate.) 
Cambrian,  absence  of  in  Mesabi  district,  198. 

iufluence  on  concentration  of  iron  ore.  263.  271 

in  Menominee  district  of  Michigan.  198. 

relations  to  Keweenawan,  198. 
Canadian  iron  districts,  glacial  erosion  of.  264. 

(.•^■fc  Ontario.) 
Canton  mine,  43. 

discovery  of  iron  ore  in.  28. 

faulting  in.  230. 

manganese  in  hard  and  soft  ores  of.  221, 


INDEX. 


305 


Canton  mine,  phosphorus  in  hard  and  soft  ores  of,  '220. 
shipment  from,  287. 
structural  relations  of  iron  ore,  228. 

transportation  of  iron  ores,  2S5. 
Carbon  of  Virginia  slate,  169-170. 

Carbonate  of  Biwabik  formation,  42, 101, 119;  described,  47 
48,  150-153. 

alteration  of,  48,  237,  238,  273. 

of  amphibolitic  chert,  138. 

of  greenalite  rock,   102,   107,   117,  119-120;   photomiero- 
g-raphs  of,  128. 

of  Gunflint  Lake,  40,  153. 

of  iron  ore  deposits,  211. 

of  Penokee-Gogebic  district,  153. 
Carbonation,  processes  of.  256. 

Carbonic  acid,  influence  in  concentration  of  iron  ore,  261- 
262. 

influence  in  formation  of  greenalite.  255-257. 
Carlton,  rocks  of,  54,  203-204. 
Caving.    { See  Mining  methods. ) 
Chamosite  compared  with  greenalite  granules,  248. 
Challenger  expedition  referred  to.  289,  253. 
Channing,  J.  P.,  referred  to,  61. 
Chert  pebbles  in  Lower  Huronian  conglomerate,  76. 

in  Pokegama  conglomerate,  98. 

{See  Ferruginous  chert.) 
Chester,  A.  H.,  referred  to,  26,27,61. 

summary  of  literature,  35. 
Chicago  mine,  amphibolitic  chert,  analysis  of,  141, 

ferruginous  chert  of  Biwabik  formation,  plate  of,  122. 

slate  in  Biwabik  formation,  146-147. 
Chippewa  quartzite,  correlation  of,  41,50. 
Chisholm  mine,  analyses  of  iron  ores  from,  214. 

shipment  from,  288. 

transportation  of  iron  ores,  2S5. 

underground  mining  in.  282. 
Chisholm  (town),  railway  connections,  23. 
Chlorite  of  Virginia  slate,  170. 
Chloritic  schists,  development  of,  68, 69, 77, 83. 

included  in  granite,  80. 

of  Archean,  63,68. 
Chubb  Lake,  Biwabik  formation  near,  52. 
Churn  drills,  300. 
Cincinnati  mine,  43,  44. 

analyses  of  greenalite  rock  from,  108-109. 

discovery  of  iron  ore  in,  28. 

ferruginous  chert  in  contact  with  quartzite,  plate  of,  126. 

photomicrographs  of  greenalite  granules,  128. 

quartzite  of  Biwabik  formation,  154. 

shipment  from,  287-288. 

slate  in  Biwabik  formation,  146-147,  175-176. 

structural  relations  of  iron  ore,  228. 

transportation  of  iron  ores,  285. 
Circulation  of  water  in  Biwabik  formation,  166. 

localization  of  iron  ores  by,  described,  265-272. 

(See  Iron  ore  deposits,  Biwabik  formation,  drainage.) 
Clarke,  F.  W.,  on  composition  of  glauconite  and  greenalite, 

243,  247. 
Clark  mine,  analyses  of  iron  ores  from,  214. 

ferruginous  chert,  analysis  of,  139-140;  plate  of,  124;  pho- 
tomicrograph of,  132, 

limonite  in,  210. 

shipment  from,  288. 

transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Clay  in  iron  ore  deposits,  219. 
Cleavage  in  Archean  rdcks,  67,  70. 

of  Lower  Huronian,  74,  77,  86. 

MON   XLIII — 03 20 


Clements,  J.  M.,  referred  to,  11,  64.  65. 159, 172, 174, 183. 
Clinton  iron  ores,  origin  of,  252;  photomicrographs  of,  250. 

similarity  of  granules  to  granules  of  Mesabi  and  Goge- 
bic districts.  128. 130,  248-252.  279. 
Colquet  (town),  rocks  near,  203,  204. 
Colquet  mine,  shipment  from,  287,  288. 
Colorado  formation  of  Upper  Cretaceous,  190. 
Columbia  mine,  analyses  of  iron  ores  from,  214. 

drainage  of,  235. 

shipment  from,  288. 
Commodore  mine,  analyses  of  iron  ores  from,  214. 

limonite  in,  210. 

shipment  from,  287. 

transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Commonwealth  iron  ores,  equivalence  to  Mesabi  iron  ores, 

37. 
Concretions  in  ferruginous  cherts  of  Biwabik  formation, 
117-118,  24S;    photomicrographs    of.   128    (specimen 
40767),  134. 

in  Clinton  iron  ores,  248;  photomicrographs  of,  250. 

in  Penokee-Gogebic  iron-bearing  formation,  117-118, 248; 
photomicrographs  of,  134  (specimens  9048  and  9625). 
Conglomerate  comparison  with  Lower  Huronian  breccia, 
86-87. 

of  Biwabik  formation,  101,  167;  described  154-159  (spec- 
mens  40851-^0855,  46020,  46021,  46026,  46031,  46032, 
46034). 

of  Biwabik  formation,  Mahoning  mine,  159. 

of  Cretaceous  formation,  189. 

of  Lower  Huronian,  described,  75-78. 

of  Pokegama  formation,  described,  94-98. 
Copper-bearing  rocks.    (See  Keweenawan.) 
Cordierite  hornstones  in  Virginiaslate formation,  described, 
171-172  (specimen  45699),  185. 
photomicrographs  of,  174  (specimen  45235). 
Correlation  of  Mesabi  series,  200-205;  summary  of,  16. 
Corsica  mine,  analyses  of  iron  ores  from,  215. 

shipment  from,  288. 

transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Cottonwood  county,  quartzite  in,  assignment  to  Potsdam,  57 
Coutchiching  series,  assignment  to  Keewatih,  53. 

origin  of,  45. 

relations  to  Keewatin,  45. 
Credner,  referred  to,  277. 
Cretaceous  series,  198;  described,  189-191;  summary,  16. 

effect  on  concentration  of  iron  ore,  263,  271. 

fossils  in  190-191  (specimens  45573,  45576,  45610,  45733). 

lignite  in.  191. 

relations  to  other  formations,  23,  49, 198. 
Croxton  mine,  analyses  of  iron  ores  from,  215. 

shipment  from,  288. 

transportation  of  iron  ores.  285. 

underground  mining  in,  282. 
Crystal  Falls  district,  ellipsoidal  greenstone  in,  64^65. 

monograph  on,  11. 

origin  of  iron  ores  in,  277. 

spilosite  from,  172,  174, 
Crystallographic  arrangement  of  quartz  particles  in  schists, 

83-84  (specimen  45492). 
Cummingtonite,  alteration  from  greenalite,  102,  117. 

of  Biwabik  formation,  141-143. 

(See  Amphiboles. ) 
Cupriferous  formation.     [See  Keweenawan.) 
Dakota,  Cretaceous  fossils  of.  190. 

quartzite,  equivalence  to  Upper  Huronian,  41,50. 
Dana,  S.  L.,  analysis  by,  241. 
Day  mine,  shipment  from,  2SS. 


306 


INDEX. 


Diiy  mine,  tmnsporation  of  iron  ores,  2So. 

underground  mining  in,  282. 
Dectien,  anaylsis  by,  241. 
Deles-iiite  of  Biwabik  formation,  119,  137. 
Denton,  F.  W.,  referred  to,  61. 
Dewalque,  analysis  by,  241. 
Diabase,  included  in  Lower  Huronian  granite,  SO.  , 

of  Keweennwan,  58;  described,  185-186. 

sills,  57,  58,  185-186. 
Diallage,  of  Duluth  gabbro,  183. 

of  Pewabic  quartzite,  51. 
Diamond  drills,  300. 
Diamond  mine,  39, 40. 

ferruginous  chert  of  Biwabik  formation,  plate  of,  124. 
Dikes  of  granite  and  porpliyry,  79. 
Diorite  included  in  Lower  Huronian  granite,  80. 

of  Archean,  03;  described,  66. 
Dip,  initial,  of  Upper  Huronian  series,  197 
Dip  of  iron  ore  deposits,  42,  225,  226. 

of  Lower  Huronian  series,  24,86. 

of  Upper  Huronian  of  Gogebic  district,  202. 

of  Upper  Huronian  series,  24,  42,  46,  51,  55,  88,  98, 165, 166, 
175,  178,  179,  202,  230,  260: 
Disappointment  Lake,  rocks  of,  52,  54. 
Docks  on  Lake  Superior,  285. 

Duluth,  Missabe  and  Northern,  view  of,  28,  298. 
Dodge,  J.  A.,  analyses  by,  223. 
Doleritesof  Archean,  63;  described,  64. 
Dolomite  in  iron  ore  deposits,  211. 
Donora  mine,  ferruginous  chert,  analyses  of,  139-140,  141. 

slate  in  Biwabik  formation,  analyses  of,  144-145. 
Dormer,  George,  indebtedness  to,  20. 
Drainage  of  Mesabi  district,  21-22. 

as  a  guide  for  exploration,  297-298. 

of  iron  ore  deposits.     {See  Iron  ore  deposits  and  Circula- 
tion.) 
Drilling,  analyses  of  sample,  300-301. 

cost  of,  300. 

in  exploration,  300. 
Duluth  and  Iron  Range  Railway,  building  of  Jlesabi  branch, 
28,  29. 

iron  ores  hauled,  28.5-2.S6. 

ownership  of,  2.S6. 

towns  reached  by,  23. 
Duluth  and  Winnipeg  Railway,  connection  with    Mesabi 

range,  28, 
Duluth,  docks  at,  285. 
Duluth  gabbro,  57,  59;  described,  182-183. 

Birch  Lake,  figure  of,  184. 

distribution  of,  57-58,  182. 

influence  on  concentration  of  iron  ore,  272-274. 

intrusive  nature  of,  59. 

metamorphism,  cau.sed  by,  50,  59,  159-164, 183-185,  272. 

petrography  of,  49,  52,  68,  59,  183. 

relations  to  Animikic  scries,  36. 

relations  to  Logan  sills,  58. 

relations  to  red  rock,  58. 

relations  to  underlying  rocks,  78,  isi,  182,  184. 

{See  ICeweenawan. ) 
Duluth  mine,  analyses  of  iron  ores  from,  215. 

depth  of,  208. 

limonite  in,  210. 

milling  in,  2.^2. 

shipment  from,  287,  2.S8. 

transportation  of  iron  ores,  285. 
Duluth,    Missabe,    and    Northern    Railway,    building    of, 
28,29. 

iron  orfcs  hauled,  285. 

ownership  of,  2.SG. 


Duluth,  Missabe,  and  Northern  Railway,  towns  reached  by, 

23. 
Duluth,  Jtissabe,  and  Northern  docks,  view  of,  28,  298. 
Dunka  River,  22, 193. 

rocks  of,  38,  60. 
Eames,  H.  H.,  referred  to,  26;  summary  of  literature.  32. 
East  Greenwood  Lake  gabbro,  57. 
Eastern  Railway  of  Minnesota,  170, 171,174. 
building  of  Hibbing  branch,  29. 
connection  with  Mesabi  range,  28. 
iron  ores  hauled  2S.5-286. 
towns  reached  by,  23. 
Elba  mine,  analyses  of  iron  ores  from,  215. 
faulting  in,  230 
limonite  in,  210. 
shipment  from,  287. 
texture  of  iron  ore  in.  224. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
Elevation  of  Lake  Superior.  21. 
Elftman,  \.  H.,  referred  t".  61,  1.59,  183,  192;  sunimary  of 

literature,  44-45,  50. 
Ellipsoidal  structure  in  Archean  basalt  of  Mesabi  district,  65. 
in  Biwabik  formation,  137. 
in  Crystal  Falls  district,  65. 
in  Vermilion  district,  65. 
Embarrass  granite,  described,  186-188;  (specimens  4-5139  and 
45075). 
age  of,  198. 

alteration  of  Biwabik  formition,  159-164. 
contact  with  Biwabik  formation,  1.59-164;  plate  of,  152. 
dikes  in  Keweenawan,  186. 

relations  to  Lower  Huronian  granite  of  Birch  Lake,  188. 
relations  to  Upper  Huronian  series,  181,  187-188,  198. 
use  of  name,  186. 
{Sec  Keweenawan.) 
Embarrass  Lake  and  River,  early  explorers  on,  21.  22,  25.  26, 

32,  36,  193,  194. 
Embarrass  station,  granite  near,  186. 
Enlargement  of  quartz  grains,  92,  94. 
Enstatite  of  Dulutli  gabbro,  183. 

of  Pewabic  quartzite,  51. 
Epidote  spherulites,  118. 
Epidote-zoisite,  alteration  from  greeualite,  117. 

in  matrix  of  ferruginous  chert,  119. 
Eveleth,  178. 

discovery  of  iron  ore  near,  28. 
railway  connections,  23. 
rocks  near,  63,  72,  73. 
textures  of  iron  ore  at,  223. 
Exploration  for  iron  pre  described,  29.V301. 
by  drilling,  300. 
by  test  pitting,  300. 
early,  of  Mesabi  district,  25-31. 
Fayal  mine,  analyses  of  iron  ores  from,  215,  217. 
delessite  in,  137. 
depth  of,  208. 
drainage  of,  236. 

ferruginotis  chert,  analyses  of,  139-140. 
glacial  erosion  of,  263. 
milling  in,  282. 

phosphorus  in  hard  and  soft  iron  ores  of,  220. 
quartzite  of  Biwabik  formation,  154;  analysis  of,  1.56. 
rocks  in  iron-ore  bo  '.ies,  212. 
shipment  from,  2''",  2.S8,  289. 
slate  in  Biwabik  formation,  147. 
steam-shovel  mining  in,  280. 
system  of  tracks  in,  282. 
texture  of  iron  ore  in.  224. 


INDEX. 


307 


Fayal  mine,  transportation  of  iron  ores,  285. 

underground  mining  in,  282. 

view  of,  292.  294. 
Fall  Lake,  Lower  Keewatin  greenstone  of,  53. 
Faircbild,  D.  L..  Indebtedness  to.  19,  20. 

referred  to,  291 . 
Faults  in  Archean  rocks,  70. 

in  Biwabik  formation,  52,  166,  179. 

in  Fokegama  formation,  179. 

In  Virginia  area,  46. 

relations  to  iron  ore  deposits,  48,  230,  233,  278. 
Felch  Mountain  area,  origin  of  iron  ores  in,  119,  277. 
Feldspar  porphyry  dikes  of  Lower  Huronian  granite,  79. 
Feldspathic  graywacke  of  Gogebic  district,  174. 
Ferric  iron  in  glauconite,  243. 

in  greenalite,  243. 
Ferrous  iron  in  glauconite,  243. 
Ferrous  silicate  in  quartzite,  92,  93. 

(See  Greenalite.) 
Ferruginous  chert  of  iron  formation,  described,  101,  116-143: 
plate  of,  122  (specimens  43027,  45588);  plate  of,  124 
(specimens  45035,  45309,  45603);  plate  of,  136  (speci- 
mens 45141,  4.5147). 

altered,  front  east  end  of  range,  160  (specimens  45057, 
45119,  45124),  163,  273  (specimens  4.5119,  45124). 

amphibolitie,  photomicrograph  of,  132  (specimen  45603), 
136  (specimens  45141,  4.5147);  analysis  of,  141  (speci- 
mens 45028,  4564S,  4.5649,  45689). 

analyses  of,  139-140  (specimens  40744,  40751,  45543,  45589, 
45590,  45596,  45603,  45653,  45662,  45672  B,  4568S,  45692, 
45694),  141  (specimens  45028,  45648, 4.5649, 45689). 

exploration  for  iron  ore  beneath,  298-299. 

granules  in,  described,  116-120;  photomicrographs  of,  128 
(specimen,  4.5419),  130  (specimen  45628),  132  (speci- 
mens 45063,  45183,  45603). 

granules  Penokee-Gogebic  district,  photomicrograph  of, 
134  (specimen  9625). 

in  contact  with  quartzite  of  Biwabik  formation,  156, 
plate  of,  126  (specimen  409.52). 

in  iron-ore  deposits,  210,  211,  219,  233. 

jaspery  phase,  Biwabik  formation,  plate  of,  126. 

phases  of,  as  a  guide  to  exploration.  299. 

relations  to  irou-ore  deposits,  227-229,  233;  figure  of,  232. 

relations  to  paint  rock,  234. 
Ferruginous  slate  of  Biwabik  formation,  101;  plate  of,  152 

(specimen  45594) , 

adjacent  to  iron-ore  deposits,  227. 
Fisher,  analysis  by,  242. 
Forellenstein  of  Duluth  gabbro,  183. 
Fort  Benton  horizon  of  Cretaceous,  190. 
Fort  Pierre  horizon  of  Cretaceous,  190. 
Fossils  of  Cretaceous  rocks,  190-191. 
Franklin  mine,  analyses  of  iron  ores  from,  215. 

paint  rock  of  Biwabik  formation,  analyses  of,  149-1.50. 

shipment  from,  287-2SS. 

transportation  of  iron  ores,  .285. 

underground  mining  in,  282. 
Fraser  Lake,  rocks  near,  52,  54. 
Furnace  use  of  iron  ores,  294. 
Gabbemichigamak  Lake,  rocks  of,  52,  53,  54. 
Garnet  of  altered  Lower  Huronian  rocks,  S3. 

of  Biwabik  formation,  160. 

of  Embarrass  granite,  187. 
Gabbro.  (See  Duluth  gabbro. ) 
Genoa  mine,  analyses  of  iron  ores  from,  215. 

depth  of,  208. 

Limonite  in,  210. 

Lower  Huronian  conglomerate  near,  72,  76,  77 

shipment  from,  287,  288. 


Genoa  mine,  texture  of  iron  ore  in,  224. 

transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Geographic  J^ames,  United  States  Board  on,  referred  to,  21. 
Geography  of  Mesabi  district,  20-21;  summary  of,  13. 
Geology,  general,  of  Lake  Superior  region,  200. 

of  Mesabi  district,  table  of  formations,  23-24:  summary 
of,  13-14. 
Georgia,  Clinton  iron  ore  from,  photomicrographs  of,  250. 
Giants  range,  178. 

constitution  of,  24, 178. 

gorges  in,  162,  193,  199. 

granite.     (See  Lower  Huronian  granite.) 

history  of  development  of,  43,  199-200. 

use  of  term,  21. 
Glacial  deposits,  described,  22,  23,  24,  49,  191-194,  199;  sum- 
mary, 16. 

effect  on  water  circulation,  269,  271. 

thickness  of,  269. 
Glacial  erosion  of  Canadian  iron  district,  264. 

of  Marquette  district,  264. 

of  Mesabi  iron  ore,  263,  271. 
Glacial  gorges  in  Giants  range,  162,  193,  199. 
Glacial  lakes  north  of  Mesabi  range,  193,  194, 199. 

(See  Lake  Norw^ood,  Lake  Dunka.) 
Glacial  strise  in  Mesabi  district,  194. 
Glauconite,  accompaniments  of,  239,  240. 

alteration  of,  30,  48,  51,  60,  278. 

analyses  of,  47,  240,  241. 

as  evidence  of  Cambrian  age,  57. 

compared  with  greenalite,  115,  239-259. 

composition  of.  61,  242,  243,  245. 

of  cretaceous,  189. 

origin  of,  48,  59,  60,  61,  189,  239,  253,  2.54. 

thickness  of  deposits,  240. 

use  of  term,  60,  247. 

volcanic  nature  of,  56. 
Glaucophane  of  Biwabik  formation,  160. 
Glenn  mine,  shipment  from,  288. 

transportation  of  ores,  285. 

underground  mining  in,  282. 
Glinka,  K.,  analyses  by,  242-244. 
Goethite  in  iron-ore  deposits,  209,  21S. 
Gogebic  district.  Monograph  on,  11. 

Gogebic  series,  concretions  in,  117-118,  128,  248,  251;  photo- 
micrograph of,  134  (specimen  9048). 

connection  with  Upper  Huronian  of  Mesabi  district, 
figpre  of,  203. 

correlation  of,  27,  35,  37,  38,  39,  41,  50,  202,  203-205,  296. 

feldspathic  grayw_acke  in,  174. 

greenalite  in,  118^  258;  photomicrograph  of.  134  (speci- 
men 9625). 

iron  ores,  comparison  with  Mesabi  iron  ores,  276;  price 
of,  293;  origin  of,  238,  277. 

siderite  in,  153. 

slate  of,  177,  180. 
Gordon,  A.  T.,  analyses  by,  144,  145, 150. 

referred  to,  189,  212,  221,  235.        " 
Grand  Marais,  Manitou  rocks  near,  58-59. 
Grand  Portage,  rocks  of,  56,  57. 
Grand  Rapids,  exploration  west  of,  204,  29( 

location  of,  22-23. 

railway  connections,  23,  28. 

thickness  of  drift  at,  192. 
Granite  of  Mesabi  district.     (See  Archean  granite.  Lower 

Huronian  granite,  and  Embarrass  granite). 
Granite  of  northern  Minnesota,  55. 
Grant  mine,  shipment  from,  288. 
transportation  of  iron  ores,  285. 


308 


INDEX. 


Grant  mine,  undersrround  mining  in,  282. 

Grant,  U.  S..  referred  to,  30.  .51, 52,  .56, 159. 160, 107,177. 183,202. 

■summary  of  literature.  59. 
Grant.  U.  S.,  and  Winehell.N.H.,  summary  of  literature,  .50. 
Graywaeke,  feldspathic  of  Gogebic  district,  174. 
of  Carlton  and  Cloquet,  203. 
of  Grand  Portage.  56,57. 
of  lower  Huronian,  described,  74-75. 
of  Virginia  slate,  109. 
Great  Northern .     (See  Eastern  Eailway  of  Minnesota. ) 
Great  Western  mine,  43. 

Green,  R.  B.,  analyses  by,  139-140, 1«-145, 156, 220. 
referred  to.  212, 221. 
screening  figures  by,  224. 
Greeualite.  described,  14,17,102-116. 

alteration  of,   116-117.132.137-138,237,238;   photomicro- 
graphs of,  128, 130. 
analyses  of  specimens,  451S0, 45758, 45765, 46766,  108-109, 

245-246. 
association  mth  iron  carbonate,  258. 
comparison  with  bavalite,  chamosite,  and  berthierine, 

248. 
comparison  with  Clinton  iron  ores,  248,  279. 
comparison  with  glauconite,  114-115,  239-247,  2.53-255. 
comparison  with  iron  carbonate.  255-259,  279. 
composition  of,  108,  242, 243-247, 256, 261;  conclusion,  113- 

115. 
explanation  of  occurrence  in  granules,  247-252. 
in  Penokee-Gogebic  series,  118. 
origin  of,  59,  239-259,  278. 

photomicrographs   of,  106  (specimen  45178);  128  (speci- 
mens 45178,  4.5765) . 
rocks  grading  to  quartzite  and  slate,  239. 
rocks,  plate  of,  104  (specimens  45176,  45647). 
specific  gravity  of.  107. 
use  of  term.  247. 
Greensand.     (See  Glauconite,  Greenalite.) 
Greenstone.     (.See  Archean.) 
Griese,  E.  T.,  referred  to,  212,  220,  223. 
Grifnn's  camp,  39. 

Groveland  formation,  granules  of.  119. 
Gruber,  J.  H..  referred  to,  291. 

Griinerite,  altered  from  greenalite  granules,  102;  photomi- 
crographs of,  128. 
of  ferruginous  chert,  119;  photomicrographsof,  132  (spec- 
imen 4.5603). 
of  iron-ore  deposits,  211. 
of  quartzite,  51,  92. 
(Sec  Amphibole.) 
Giimbel,  analyses  by,  242. 
Gunflint  iron  formation,  correlation  of,  27,  34. 

origin  of  iron  ores,  37,  277. 
Gunflint  Lake,  analyses  of  magnetite  from,  221, 
carbonates  at,  40,  56,  1.53. 
early  e.xrilorers  on,  25,  26,  32. 
iron  formation,  alteration  of,  159,  160. 
litcnitvire  on,  61. 
location  of,  25. 

rocks  near,  27,  40,  62,  63,  1.59,  169, 177, 183.  185,  201. 
Hale  mine,  43,  44. 

analyses  of  iron  ores  from,  215. 

dip  of  iron-ore  deposit  layers,  225. 

discovery  of  iron  ore  in,  28. 

faulting  at,  166,  230. 

limouite  in,  210. 

rocks  adjacent  to  iron-ore  deposits  in.  227. 

shipments  from,  2.S7,  288. 

steam-shovel  mining  in.  2.S0. 

strtictunil  reliitituiw  of  iron  ore  of.  229. 


Hale  mine,  transportation  of  iron  ores.  285. 

view  of,  294, 
Hammond,  A.  J.,  analysis  by,  139-140. 
Hampe,  analyses  by,  262. 
Harker,  referred  to,  172. 
Hanshof er,  analyses  by,  241 . 
Hawkins  mine,  railway  connections,  23. 
shipment  from,  288. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
Head,  A.  P.,  referred  to,  62. 
Head,  Jeremiah,  referred  to,  62. 
Heddle,  F.,  anaysis  by,  241. 
Hematite  in  iron-ore  deposits,  209,  218. 
in  quartzite,  93. 
pore  space  in,  224. 
(.See  Iron  ore.) 
Hibbing,  analysis  of  water  west  of,  264. 

Archean,  Lower  Huronian,  Upper  Huronian  rocks  north- 
west of,  63,  64,  67,  69,  71,  72,  80,  84. 
discovery  of  iron  ore  near,  28. 
iron-ore  deposits  near,  206. 
railway  connections,  23. 
texture  of  ore  at,  224. 
Hinckley,  sandstone,  57. 
Hinsdale,  explorations  near,  39. 
Hornblende.     (See  Amphibole.) 
Hornblendic  schists  of  Archean,  63;  described,  66-68. 

included  in  Lower  Huronian  granite,  SO. 
Hornblende  granite  of  Archean.    (See  Archean.) 
Horses  of  rock  in  iron-ore  deposits  explained  by  circulation 
of  water,  269. 
view  of,  294. 
History  of  district,  2-5-31. 
Houghton,  referred  to,  277. 
Hubbard,  referred  to,  277. 
Hudson  Bay,  22,  25. 

Hull  mine,  analyses  of  iron  ores  from.  215. 
analysis  of  water  from,  264. 
depth  of,  208. 

transportation  of  iron  ores.  285. 
underground  mining  in,  282. 
Hulst.  N.  P.,  referred  to.  62. 
Hunt,  analyses  by,  241. 

referred  to.  277.  ^ 

Hunters  Island,  Lower  Keewatin  greenstone  of.  53. 
Huronian,  use  of  term,  202. 

(Sec  Upper  Huronian,  Lower  Huronian. ) 
Hyperstheno  of  contact  rocks  of  gabbro.  51.  .59.  160. 

of  Duluth  gabbro,  183. 
Ilmenite,  of  altered  Lower  Huronian  rocks,  S3, 
of  Embarrass  granite,  187. 
of  Keweenawan  diabase,  185. 
Iron  bacteria  procijiitating  hydrated  iron.  256. 
Iron-bearing  formation.     (See  Biwabik  formation.) 
Iron  carbonate  alteration  from  glauconite,  60. 
alteration  from  greenalite,  279. 
altering  to  iron  ores,  37,  263,  277,  278. 
association  with  greenalite  rocks,  258. 
comparison  of  development  with  greenalite.  255.  279. 
concretions  in,  248. 
(.See  Carbonate.) 
Iron  Lake,  analyses  of  magnetite  from,  222,  223. 
contact  of  iron  formation  and  syenite.  :3S. 
Pewabic  quartzite  of,  51. 
Iron  ore.  absence  in  oast  end  of  Mesabi  range.  207,  272- 
274, 
absence  under  N'irginia  slate,  297. 
abstraction  of  silica  from.  2li2.  264. 


INDEX. 


309 


Iron  ore,  cTiemistry  of,  26,  212-223. 

comparison  with  otlier  Lake  Superior  ores,  296. 

correlation  of.    (See  Biwabik  formation.) 

covered  by  ferruginous  chert,  298-299. 

cubic  feet  per  ton,  224. 

deposits  40, 48:  described,  206-236;  summary,  16-17. 

discovery  of  in  Mesabi  district,  39,  62. 

early  explorers  on,  33. 

elevation  of,  297. 

exploration  for,  295-300. 

furnace,  use  of,  294. 

glacial  erosion  of,  263,  271. 

locahzation  by  water  circulation,  279;  described,  265- 

272. 
magnetic.    {See  Magnetic  attraction.  Magnetite.") 
migration  of,  figure  270. 
mixture  with  old  range  iron  ores,  294. 
not  mined,  290. 
of  Lake  Superior  region,  origin  of,  61;  paper  on,  19; 

price  of,  293-294. 
of  Minnesota,  horizons  of,  38. 
of  Vermilion  district,  38,  40,  53. 
of  west  end  of  range,  211. 
origin  of,  37,  40,  42,  43,  44,  48,  51,  56,  60.  237,  238,  260-272, 

277-279;  described,  56,   237-279;   summary    of,  16-17. 

42,  263,  277. 
o^vnership  of,  291-292. 
phosphorus  in,  219,  274. 
pitch  of  deposits,  276. 
pore  space  in,  262. 
possibility  of  finding  in  Lower  Huronian  and  Archean 

of  Mesabi,  299. 
jjroperties,  mapping  of,  291. 

(.S'ee  Mines.) 
proportion  of  area  to  that  of  Biwablk  formation,  101, 

20L-,  296. 
proportion  of  Bessemer,  219,  290. 
relations  to  adjacent  rocks,  40,  48,  227,  234,  278,  299. 
reserve  tonnage  in  Mesabi  district,  290. 
reserve  tonnage  in  old  ranges,  290,291. 
sand  in,  211,296. 
shipment  by  railways,  285. 
shipment  from  Mesabi,  29, 287-289. 
shipment  from  Michigan,  289. 
shipment  from  Minnesota,  289. 
shipment  from  United  States,  289. 
Iron  pyrites  altering  from  greenalite  grantiles,  138. 
in  ferruginous  chert  of  Biwabik  formation,  138. 
in  iron  ore  deposits,  211,219. 
in  Lower  Huronian  sediments,  75. 
in  quartzite,  92. 
Iron  Trade  Review,  referred  to,  29, 287. 
Irving,  R .  D. ,  referred  to,  27, 42,  44, 45, 183,  200, 201, 2.55, 277, 278: 

summary  of  literature,  34. 
Isle  Royal,  Puckwunge  conglomerate  at,  57. 
Itasca  or  10th  moraine,  49,  192. 

(See  Moraine.) 
Jasper  in  Biwabik  conglomerate.  157-158. 

in  Biwabik  formation,  120,  157, 158;  plate  of,  126  (speci- 
men 45420). 
in  Gogebic  district,  origin  of,  272. 
in  Lower  Huronian  conglomerate,  77. 
in  Pokegama  conglomerate,  97,  98. 
Johnson,  E.  J.,  analysis  by,  150. 

referred  to,  212. 
Joints  in  Archean  rocks,  70. 

in  Biwabik  formation,  48,  166,  179. 

in  gabbro,  182. 

in  iron  ore  deposits,  226-227,  230,  233;  filled  -with  vein 

quartz,  212. 


Joints  in  Lower  HurOnian,  86. 
in  Oliver  mine,  view  of,  284. 
in  Pokegama  formation,  99,  179. 

influence  on  water  circulation  and  concentration  of  iron 
ore,  265.  267,  269,  271,  279. 
Jordan  mine,  analyses  of  iron  ores  from,  215. 
mining  in,  282. 

shipment  from,  288.  , 

transportation  of.iron  ores,  2S5. 
Kabetogoma  Lake,  granite  of,  intrnsion  in  Lower  Keewatin, 

bb. 
Kakabikag  Falls,  use  of  term,  37. 
I   Kame  gravels  of  Pleistocene,  193. 
Kanawha  mine.  43. 

analysis  of  iron  ores  from,  215. 
boundaries  of,  291. 
•  dip  at,  179. 
dip  of  iron  ore  deposit  layers,  225. 
discovery  of  iron  ore  in,  28. 
faulting  in,  166, 230. 
shipment  from,  288. 
structural  relations  of  iron  ore  of,  229. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
I   Kaolin  in  iron  ore  deposits,  212. 

Kawishiwin  greenstone  of  Lower  Keewatin  described.  53. 
I  Keewatin,  use  of  term,  202. 
,   Keewatin  series.  40,  41,  46,  52,  53-54,  201. 
correlation  of,  41,  201-202. 
greenstone,  alteration  to  gabbro,  58. 
mapping  of,  52. 

relations  to  other  series,  45,  46,  53,  54,  55,  58. 
schists,  origin  of,  45. 
Kekequabic  Lake,  granite  of,  55. 
Kettle  River,  Hinckley  sandstone  of.  57. 
Keweenawan  series,  described,  57-59,  182-188;  summary  of, 
15-16. 
correlation  with  Lower  Cambrian,  52-53. 
folded  with  Upper  Huronian,  260. 
relations  to  other  series,  23,  39,  42,  43.  45,  46,  49,  56,  57-58, 

197-198,  200,  260,  272. 
{See  Embarrass  granite,  Duluth  gabbro.) 
Kimball,  referred  to,  277. 
Kimberly,  Minn.,  greenstone  near,  203. 
magnetic  attraction  near.  204. 
quartzite  near,  203,  204. 
Knerr,  analysis  by,  242. 
Knife  Lake  formation,  23. 
Kupffer,  A.,  analyses  by,  242. 
Labradorite  of  Duluth  gabbro,  183. 
La  Belle  mine,  shipment  from,  288. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
Lac  la  Croix,  granite  of,  55. 
Lacroix,  referred  to,  248. 

Lake.    (See  Akeley,  Basswood,  Birch,  Burntside,  Chubb,  Em- 
barrass, Rail,  Fraser.  Gabbemiehlgamak,  Gunfiint, 
Iron,  Kabetogoma,  Kekequabic,  Muskrat,    Rainy, 
Saganaga,  Thomas,  White  Iron.) 
Lake  Dunka,  194. 
Lake  Huron,  original  Huronian  rocks  of,  correlation  with 

Upper  Huronian,  27. 
Lake  Norwood,  194. 

Lake  Ogishke-Muncie.  Lower  Keewatin  of,  54. 
Lake  Superior,  docks  at,  285. 

elevation  of,  21. 
Lake  Superior  Consolidated  Iron  Mines  Company,  indebted- 
ness to,  20. 
Lake  Superior  lobe  of  ice  sheet,  194. 


310 


INDEX. 


Lake  Superior  mines,  43. 

(inalyses  of  iron  ores  from.  215,  217. 
shi[>ment  from.  2S7.  2.SS. 
Lake  Superior  region,  general  geology  of,  200. 
monograph  on  geology  of.  11. 
sketch  map  of,  19. 
Lane  and  Sharple.ss,  referred  to.  H2. 
Lanners.  T.  L.,  referred  to.  02. 
Laura  mine,  shipment  from.  2ss. 
transportation  of  iron  ores.  28.'.. 
underground  mining  in.  2S2. 
Laurentian  series,  assignment  to  Keewatin,  53. 

distribution  of.  -10. 
Lawson.  referred  to,  .58. 
Leetonia  mine,  shipment  from.  2S.S. 
steam  shovel  mining  in.  280. 
transportation  of  iron  ores,  285. 
Leith,  C.  K..  referred  to,  31,  243,  245. 

Leith.  C.  K..  and  Van  Hise.  C.  R..  summary  of  literature,  60. 
Lerch  Brothers,  analyses  hy,  139-140.  114-145,  1.50. 

referred  to,  212,  223. 
Lignite  in  Cretaceous,  191. 
Limestone  in  Biwabik  formation  described,  150-153. 

of  Virginia  slate.  171  (specimen  45464),  46,  56. 
Limonite,  alteration  from  greenalite,  117. 
in  iron-ore  deposits,  209-210,  218-219. 
pebbles  in  Biwabik  conglomerate,  157-1.58. 
pore  space  in,  224. 
Limonltic  chert  in   Biwabik  formation,  137;  plate  of,  124 

(specimen  45035). 
Lincoln  mine,  shipment  from,  288. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
Literature  of  Mesabi  district,  25-31,  61-62. 
Little  Falls,  mica-schist  at,  56. 
Little  Marais,  Puckwunge  conglomerate  at,  57. 
Little  Kocky  Tails,  use  of  term,  37. 
Locke,  referred  to,  237. 
Logan  sills.  58. 
Lone  Jack  mine,  43. 

Lon,i;year,  E.  J.,  referred  to,  62,  99,  154, 166, 186,  300. 
Longyear  mine,  analyses  of  iron  ores  from,  216. 

shipment  from,  288. 
transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Lower  Cambrian,  correlation  with  Animikie  and  Keweena- 
wan,  36,  43,  52-53. 
use  of  term,  27. 
Lower  Huronian  granite,  36,41,45,72,186,188,196;  described. 
7.S-.S0. 
contact  with  gabbro  of  Birch  Lake,  78;  figure  of,  184, 
inclusions  in,  80. 

relations  to  other  rocks,  41,  45,  M.  55,  70,  71,  ,84.  l.ss. 
variety  of,  85. 
vein  quartz  at  contact,  S3, 
Lower  Huronian  porpliyries.     (.sVi  Porj.liyritie  rliyolite  of 

Lower  Huronian.) 
Lower  Huronian  series  described.  72-.S7  (specimen  4.5494): 
summary  of,  13-14. 
alteration  by  granite,  67;  photomicrograph  of,  82  (speci- 
mens  45112,    45414.  4,5415,  4.5410),    83-84   (specimen 
45114). 
as  a  source  of  iron  for  iron  f(prmation,  255. 
deposition  of,  195. 

distribution  of,  24,  60,  66,  70,  72,  73,  80, 
equivalence  to  Ogishke  and  Knife  Lake  fornialions,  2:!, 
erosion  of,  195, 
in  Arcliean  area,  o:i-6", 
near  Carlton  and  Cloquet,  203, 


Lower  Huronian  series  near  Mississippi  River,  204, 
ol  Lake  Superior  region,  200. 
possibility  of  iron  ores  in.  299. 
relations  to  other  series,  23,  70,  71,  84-86,  195,  196. 
vein  quartz  in,  83. 
Lower  Keewatin.     (.S'ee  Keewatin.  i 
Magnetic  attraction  in  exploration.  164-165,  295,  296. 
near  Kimberly,  204. 
west  of  Grand  Rapids,  204. 
Magnetite,  alteration  from  greenalite.  117.  238. 
analysis  of,  221,  222. 
of  altered  Lower  Huronian  rocks,  S3, 
of  Biwabik  formation  near  gabbro,  26,  27,  35,  42,  44,  49. 
1.59,  272,  273:  photomicrograph  of,  136:  summary  of, 
17-18. 
of  Duluth  gabbro,  183:  analysis  of,  223. 
of  iron  ore  deposits,  209,  211. 
of  Keweenawan  diabase,  185, 
of  Pewabic  quartzite,  51, 
of  sideritic  slate  of  Birch  Lake,  153. 
origin  of,  49,  117,  238. 
Manganese  in  iron  ore  deposits.  212,  220,  221. 
Manitou  division  of  Keweenawan,  57,  58. 
Manitou  River,  Puckwunge  conglomerate  on,  57. 
Mallet,  analyses  by,  242. 
Mailman,  referred  to,  40. 
Mailman  Camp,  augite  granite  near,  78. 

rocks  near,  39,  40,  147,  295;  sketch,  SO. 
Malta  mine,  analyses  of  iron  ores  from,  214,  216. 
depth  of,  208. 
shipment  from,  2SS, 
steam-shovel  mining  in,  280, 
transportation  of  iron  ores,  285. 
Mahoning  mine,  analyses  of  iron  ores  from.  214.  216. 
analysis  of  ferruginous  chert,  139, 140. 
articles  on,  referred  to,  61,  62, 
conglomerate  in,  101,  159. 
depth  of,  208. 

ferruginous  chert  of  Biwabik  formation,  p'ate  of,  122. 
limonite  in,  210. 
paint  rock  of  Biwabik  formation,  analysis  of,  149-150: 

plate  of,  152. 
plan  of  tracks,  280,  281. 
shipment  from,  287-289. 
steam  shovel  mining  in,  2S0,  281. 
transportation  of  iron  ores,  285. 
view  of,  296. 
Map  of  Jlesabi  district  by  U.  S,  Geological  Survey,  in  pocket, 

by  Jlinnesota  Survey,  referred  to,  40,  -50.  60. 
Marck,  referred  to,  241. 

Mariska,  Lower  Huronian  conglomerate  near,  7.5-77,  86. 
Marquette  district,  Arehean  sediments  in.  77. 
erosion  of,  2t>4, 
griincrite-schist  from,  142, 
monograph  on,  11. 
Marquette  iron  ores,  apatite  in,  275. 
comparison  witli  Mesabi  ores,  276. 
origin  of,  277. 
price  of,  293-294. 
sliipmelit  t)f,  289. 
Maltliow.  referred  to,  57, 
MoKaskill.  ,lohn,  referred  to,  28. 
McKinley,  178. 

.•\rchean  rocks  north  of,  63.  • 

discover}'  of  iron  ore  near,  2S. 
mine,  43,  44. 

railway  connections.  23. 
Meadow  exiilonition.  analysis  of  slaie  from,  170. 
Meeds,  A.  1>..  analysis  by.  139,  140,  144,  145. 


INDEX. 


311 


MelviUe,  W.  H..  analysis  by.  222. 
Menominee  district,  Cambrian  in.  198. 
monograph  on.  11. 
origin  of  iron  ores  in,  277. 
shipment  from.  289. 
Menominee  series,  equivalence  to  Animikie  series.  41. 
Merritts.  referred  to  27,  29,  40. 

Mesaba  station,  contract  of  Lower  Huronian  sediments  and 
granites  north  of,  85. 
exploration  near,  40. 
ferruginous  chert  with  granules,  photomicrograph  of, 

128  {specimen  45419) . 
railway  connections  of,  28. 
Mesabi  Syndicate  Company.  44. 
Mesabi  Chief  mine,  ferruginous  chert,  analysis  of  139,  140 

(specimen  45543). 
IMesabi.  or  Eleventh  moraine,  49,  192. 
Mesabi  range,  divisions  of,  46. 
elevation  of,  21. 
shipments  from,  29. 
use  of  term,  21,  25-26,  182. 
{See  Giants  range.) 
Mesozoic,  absence  of  in  Mesabi  district.  198. 
Metabasalts.     {Sec  Basalts.) 
Metadolerites.     {See  Dolerites. ) 
Mica  in  iron  ore  deposits,  211. 

(S'eeBiotite.) 
Micaceous  quartz-slate  of  Pokegama  formation,  described, 
93.  94. 
{Ste  Pokegama  quartzite.) 
Mica-schists  of  Archean,  63;  described,  68. 
development  of,  68,  69,  77,  83. 
included  in  granite,  80. 
of  Animikie,  56. 
Michigan  Iron  ores,  apatite  in.  275. 

comparison  "with  Mesabi  iron  ores,  276. 
price  of,  293,  294. 
shipment  of,  289. 
reserve  tonnage  of,  290,  291. 

{Sec  Penokee-Gogebic,  Marquette,  Crystal  Falls,  Felch 
Mountain,  Menominee.) 
Michipicoten  district,  origin  of  iron  ores  in,  277. 
Milling  at  Fayal  mine,  described.  282. 

view  of,  294. 
Mines.     {See   Adams,    iEtna,    Agnew,    Arcturus,    Auburn, 
Biwabik,  Burt,  Canton,  Chicago,  Chisholm,  Cincin- 
nati. Clark,  Columbia,  Commodore,  Corsica,  Croxton, 
Day,  Donora,  Duluth,  Elba,  Fayal,  Franklin.  Genoa. 
Glenn,  Grant,  Great  Western,  Hale.  Hawkins,  Hull. 
Jordan,  Kanawha,  Kimberly,  Lake  Superior  mines, 
Laura,    Leetonia,    Lincoln,    Longyear.    Mahoning, 
Malta,  Minnewas,  Minorca,  Moose,  Morrow.  Moss. 
Mountain  Iron,  Norman,  Ohio,  Oliver,  Paddock's, 
Pearce,  Penobscot.  Pettit.  Pillsbury,  Roberts,  Rouch- 
leau,    Sauntry-Alpena,    Security,    Sellers,    Sharon, 
Sparta,  Spruce,  Stephens,  Stevenson.  Union,  Utica, 
Victoria,  Virginia,  Williams,  Wills,  Wyoming.) 
Mines,  not  shipping,  290. 
Mining  methods,  43,  61-62;  described,  280-285. 
Minnesota  iron  ore,  price  of,  293,  294. 
reserve  tonnage  of,  290,  291. 
shipments  of,  289. 
Minnesota  river,  rocks  of.  41,  50,  53. 
Minnesota  Survey,  Final  Report  of,  30. 
maps  referred  to,  30,  40,  50,  60. 
{See  Winchell.  Grant,  Upbam,  Elftman,  Spurr.) 
Minnewas  mine,  shipment  from,  287,  288. 
Minorca  mine,  analyses  of  iron  ores  from,  216. 
milling  in,  2S2. 


Minorca  mine,  shipment  from,  288. 

transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Missabe  Mountain  mine,  43,  44. 
Mississippi  and  Xorthern  Railway,  29. 
Mississippi  River,  21,  22,  25. 

Keewatin  sediments  of,  53,  54. 

mica-schists  on,  56. 

quartzite  on,  90,  92. 
Missouri  zinc  ores,  comparison  with  development  of  green- 

alite  and  carbonate,  258,  259. 
Moose  mine,  manganese  in,  220. 
Moose  Track  mine,  glacial  origin  of,  263. 
Moraine,  Itasca  or  10th,  49,  192. 

Mesabi  or  11th,  49, 192. 
Morrison  County.  Lower  Keewatin  of,  54. 

mica-schists  in,  56. 
Morrow  mine,  analyses  of  iron  ores  from,  216. 

milling  in,  282. 

shipment  from,  288. 

transportation  of  iron  ores,  285. 
Moss  mine,  slate  in  Biwabik  formation,  analysis  of,  144, 145 

(specimen  45561). 
Mountain  Iron  mine,  analyses  of  iron  ores  from,  215,  216. 

analyses  of  ferruginous  slate  from,  144,  145  (specimen 
45645). 

analyses  of  paint  rock  from,  149.  150. 

comparison  of  grades  in,  219. 

depth  of,  208. 

development  of  iron  ores  from  carbonate,  238. 

dips  at,  175,  179,  225. 

discovery  of  iron  ore  in,  28.  29,  40. 

drainage  of,  235,  236. 

ferruginous  chert  of  Biwabik  formation,  plate  of,  124 
(specimen  45035). 

gradation  of  iron  ore  to  ferruginous  chert,  233. 

manganese  in,  220,  221. 

magnetite  near,  42. 

plan  of  tracks,  280,  281. 

railway  connections,  23. 

rocks  north  of.  63,  66,  67,  69,  70,  71.  72.  78,  79,  SO,  84,  85, 
96:  sketch,  70. 

shipment  from,  287,  288,  289. 

slate  in  Biwabik  formation,  analysis  of,  144,  145  (speci- 
men 45645). 

steam-shovel  mining  in,  280,  281. 

texture  of  iron  ore  in,  224. 

transportation  of  iron  ores,  285. 

view  of,  286. 

views  of  contact  of  iron  ore  with  wall  rock,  232. 

water  in,  218,  219,  235,  236. 
Murray  referred  to,  239,  242.  253.  254,  255,  259. 
Muscovadite,  association  with  Pewabie  quartzite,  51. 

use  of  term,  58. 
Muskrat  Lake,  Biwabik  formation  near,  52. 
Nashwauk  town.  Archean  rocks  north  of,  63. 
Nelson  River,  21,  22,  25. 
New  Brunswick,  St.  Johns  group  of,  equivalence  to  iron 

ore,  57. 
New  England  City,  Clinton  iron  ore  from,  photomicrographs 

of,  250. 
New  England  mine,  43. 
Newman,  indebtedness  to.  19. 
New  Ulm,  Puckwunge  conglomerate  at,  57. 

quartzite  at,  assignment  to  Potsdam,  57. 
New  York,  photomicrographs  of  Clinton  iron  ore  from,  250. 
Nichols,  J.  A.,  referred  to,  27. 
Nicollet,  J,  N.,  referred  to,  25. 

summary  of  literature,  32. 


312 


INDEX. 


Sodules  in  Biwabik  formation,  137. 
in  iron-ore  deposits,  226. 
in  Virginia  slate.  137. 
Norman  mine,  milling  in.  2.S2. 

phosphorus  in  hard  and  soft  iron  ores  of.  220. 
shipment  from,  287-288. 
texture  of  iron  ore  in,  224. 
Norwood.  J.  G.,  referred  to,  25.  26. 

summary  of  literature,  32.  ^ 

Oeher.  yellow,     (.S'n  Limonite,) 
Ogishke  formation,  equivalence  to  Lower  Huronian,  23. 

(See  Lake  Ogishke.) 
Ohio  mine,  43. 

shipment  from,  287-288. 
Okwanin,  gahbro  near,  36. 

(.Sff  Allen  .Junction). 
Olcott,  W. .!.,  indebtedness  to,  20. 
Oliver  mine,  analyses  of  iron  ores  from,  215,  216. 
Cretaceous  rocks  in,  189. 
dip  at,  179,  225,  226. 
faulting  in,  230. 

ferruginous  chert,  analysis  of,  139-140  (specimen  40744) 
limonite  in,  210. 
manganese  in,  220,  221. 
nodules  in  Biwabik  formation,  137. 
phosphorus  in  limonite,  275. 
shipment  from,  267,  288. 

structural  relations  of  iron-ore  deposits,  227,  228. 
steam-shovel  mining  in,  280. 
transportation  of  iron  ores,  285. 
view  of  wall  rock  showing  jointing,  284. 
views  of,  284. 
Olivine  in  contact  rocks  of  gabbro,  45,  51,  .59. 160. 

in  Duluth  gabbro,  183. 
Ontario,  western,  correlation  of  Keewatin,  41,  50. 
Oolitic  structure  in  Clinton  iron  ores,  photomicrographs  of, 
250. 
in  Mesabi  iron  ores,  248. 
Ophitic  texture  in  dolerites,  64. 
Original    Huronian    beds,  correlation  ivith  Animikie  and 

Upper  Huronian,  27,  34. 
Organic  matter  in  ferruginous  cherts,  140. 

in  iron  ore,  218, 
Paddock's  mine,  43. 

Paintrock.lOl;  described,  149;  plate  of,  152  (specimen  4.5.587). 
alteration  from  slate,  169,  234. 
analyses  of,  149-150  (specimens  40661,  45594,  46646), 
composition  of,  149-150  (specimens  40661,  45594,  45646), 

223, 
effect  of  flowage  of  water,  265-266. 
relations  to  iron-ore  deposits,  212,  223,  227,  234,  263, 
Paleozoic,  absence  of,  in  Mesabi  district,  198. 
Partridge  River,  21. 

Pearce  mine,  analyses  of  iron  ores  from,  216. 
shipment  from,  288. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
Pegmatitic  dikes  in  Lower  Huronian  granite,  79, 
Penobscot  mine,  alteration  of  slate  to  paint  rock,  149,  234. 
analyses  of  iron  ores  from,  210. 
drainage  of,  236, 

ferraginous  chert,  analysis  of,  139-140  (specimen  45696). 
ferruginous  slate  of  Biwabik  formation,  plate  of,  152 

(specimens  .15592,  45.594). 
paint  rock  of  Biwabik   formation,  analysis  of,  149-160 

(specimen  45.59'l). 
shipment  from,  287-2.88. 

slate  in   Biwabik  formation,  146;    analysis  of,  144-145 
(specimen  4,5.591);  plate  of,  1,")2  (specimen  45594),  146, 


Penobscot  mine,  transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Penokee-Gogebie.    (See  Gogebic.) 
Peridotite  of  Archean  described,  66. 
Perknite  of  Archean,  66. 
Pettit  mine,  shipment  from,  288. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
Pewabic  quartzlte,  39,  40, 42,  43,  45,  46,  51,  52,  .51,  60. 

{See  Pokegama  quartzite.) 
Phosphorus,  comparison  in  hard  and  soft  iron  ores,  220. 
determination  in  drill  samples,  300-301. 
in  ferruginous  chert,  140. 
in  hematite,  219. 

in  iron-ore  deposits,  211,  219,  220,  274.' 
in  limonite,  220. 
in  paint  rock,  223. 
precipitated  by  alumina,  275. 
solubility  of,  275. 
source  of,  274. 
summary  of,  18. 
Pike  Rapids,  Lower  Keewatin  of,  54. 
Pike,  Z.  JI.,  referred  to,  25. 

summary  of  report  by.  31. 
Pike  liiver,  22. 

granite  of,  36. 
Pillsbury  mine,  analyses  of  iron  ores  from,  216. 
depth  of,  208. 
shipment  from,  288. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
Pine  County,  Hinckley  sandstone  of,  57. 
Pine  of  Mesabi  district,  22. 

Pipestone  County,  quartzite  in,  assignment  to  Potsdam,  67. 
Pipestone  Rapids,  Lower  Keewatin  of,  53. 
Pisani,  analysis  by,  241. 
Pitch  of  iron-ore  deposits,  207-208. 
Pleistocene.    (.8cf  Glacial  deposits.) 
Pokegama  Falls,  37, 38, 39, 92, 154. 
early  explorers  on,  25, 26. 31-33. 
stria;  at,  194. 
Pokegama  quartzite  described,  55, 90-99;  summary  of,  14, 
as  a  guide  to  exploration,  298. 
at  Pokegama  Falls,  92, 
correlation  of,  42. 
deposition  of,  196. 

influence  on  water  circulation,  266, 267. 
metamorphism  of,  43, 
near  Aitkcn,  Minn.,  203. 
near  Kimberly,  Minn., 203-204, 
relations  to  iron-ore  deposits,  227, 230, 278. 
relations  to  other  formations,  42, 51, 55, 99, 196, 230. 
thickness  of,  55, 99,  ISO, 
{Sec  Pewabic  quartzite.) 
Porphyries  of  Vermilion  district,  6.8-69. 
Porphyritic  rhyolite  of  Archean  described,  68-69. 

of  Lower  Huronian  described,  78-80. 
Potash  in  glauconite,  17,243,247,2.54. 

ingrcenalitc,243,247. 
Potsdam  formation,  reference  of  Pokegama  quartzite  to,  33. 
Potsdam,  use  of  term,  57. 
Prairie  River,  21,  22,  25,  193. 

Prairie  River  Falls,  early  explorers  on,  26,  33,  37,  38. 
rocks  at,  79,  90,  94,  1,54, 
sketch  of,  91. 
stri:e  at,  194. 
Production,    (.svr  Shipments.) 
I'uckwungc  conglomerate,  56,  57. 
Pyrites.     (See  Iron  i>yrites.) 


INDEX. 


313 


Pyrolusite  in  iron-ore  deposits,  211,  212. 
Quartz-biotite-schist  of  Animikie,  45. 
Quartz,  enlargement  of,  92,  94. 

in  iron-ore  deposits,  211. 
Quartz-slate  of  Pokegama  formation,  relations  to  massive 

quartzite,  94. 
Quartzite,  Biwabik,  described,    154-159  (specimens    40S51- 
40855,   45119,  45665,  45668,    45687,  45753,  46020,  46021, 
46026,  46031,  46032,  46034,  46070). 
Pokegama,  described,  90-99. 
(See  Biwabik  quartzite,  Pokegama  quartzite,) 
Quartzite.     (.See  Pokegama,  Pewabic,  Baraboo,  Cliippewa 

Falls,  Barron  County,  Sioux,  South  Dakota.) 
Railways  of  Mesabi  district,  23,  28. 

(See  Great  Northern  Railway,  Eastern  Railway  of  Min- 
nesota, Duluth  and  Iron  Range  Railway,  Duluth, 
Missabe  and  Northern  Railway,  Duluth  and  Win- 
nipeg Railway,  Mississippi  and  Northern  Railway.) 
Rainy  Lake,  25. 

Keewatin  rocks  of,  53. 
Range   1  W.,T.  63  N.,  58. 
2E.,  T.  64  N.,  ,57. 
2W.,T.  62N.,5S. 

3  W.,  T.  65  N.,  section  36,  223, 

4  W.,  T,  62  N.,  58. 

iW.,  T.  65  N.,  section  27,  54. 
.section  29,  221. 

5  W.,  T.  65  N.,  section  34,  64. 
6W.,  T.  60  N.,  58. 

7  W.,  T.  60N.,6S. 
10  W.,  T.  58  N.,  58. 
10  W.,  T.  62  N.,  section  30,  54,  221, 

10  W.,  T.  63  N.,  section  36,  223. 

11  W.,T.  65N.,  58. 

11  W.,  T.  63  N.,  37. 

12  W.,  51,  147,  1.59,  163. 

(See  Embarrass  granite,  Duluth  gabbro,  Biwabik 
formation.) 
12  W.,  T.  60  N.,  44-^5. 

section  3,  136. 

section  17,  136,  152,  160. 162, 187, 193. 
12  W.,  T.  61  N.,  section  34,  222. 

section  35,  38.  • 

,  147,  1.59-163,  182,  186. 


13  W., 
13  \V., 


T.  59  N.,  sections,  168. 
section  8, 168. 
section  34,  186. 

13  W.,  T.  60  N.,  section  5, 185. 

section  8, 185. 
section  22, 132. 
section  23,  222. 
section  25,  168,  185. 
section  29,  90,  138, 154. 
section  32,  44,  90. 
section  35,  185. 
14W.,  26-27,  147,  182. 

14  W.,  T.  59  N.,  34,  37. 

section  1,  147. 

section  2,  186, 188. 

section  8,  193. 

section  9,  82,  85, 193, 

section  11,  40,  72,  79-80. 

section  14,  34. 

section  15,  34, 147. 

section  16,  79,  82,  147,  193, 

section  17, 193. 

section  18,  85,  94,  95, 154. 

section  19, 147. 

section  21, 126,  139-140,  147,  172. 


Range  14  W.,  T.  59  N, 


14W.,T.  60  N, 
15  W.,  T.  58  N. 
15  W.,  T.  59  N. 


16  W.,  T.  58  N. 


16  W.,  T.  59  N.^ 


17  W,,  T.  57  N. 
17  W.,  T.  58  N. 


17  W.,  T.  59  N"., 
18W.,T.  58N., 

18W.,T.  59N., 


19  W.,  T.  58  N. 


20  W.,  T.  58  N, 

21  W.,  T.  57  N. 
21  W.,  T.  58  N. 


22  W.,  71. 

22  W.,T.  57N. 


23  W.,  71. 

23  W..  T.  56  N. 


,  section  22, 147. 

section  27, 174. 

section  28,  34. 

section  34, 166,  186. 
,  34,  37. 

,  section  3,  119, 169, 170. 
,  section  22,  104, 106, 108-109,  128, 147. 
section  26, 147. 
section  28, 132,  141,  147. 
section  33,  66,  93,  96. 
section  35,  108. 
section  2,  230. 
section  3,  73,  79,  86,  96. 
section  4,  122,  141,  147,  154,  155. 
section  5, 1.55. 
section  6,  69. 
section  7, 193. 
section  34,  86. 
section  36,  65. 
,  section  6, 147. 

section  3,  89, 139-140, 154, 155, 156. 
section  7,  66. 
section  8,  146. 
section  9,  146. 
section  13, 154,  155. 
section  15,  52. 
section  16,  69. 
section  20,  7.5-76,  90. 
section  21,  69,  75-76. 
section  22,  52,  68,  75-76,  86. 
section  24,  147. 
section  28,  139-140. 
section  29,  7-5-76. 
section  32,  87. 
section  33,  139-140,  155. 
section  34,  77. 
section  32,  154. 
section  35, 128. 
section,  3,  28. 
section  7, 144-145,  146. 
section  8,  146. 
section  18,  139-140. 
section  22,  72. 
section  23,  72. 
section  25,  69,  72. 
section  27,  72. 
section  28,  72. 
section  30,  193. 
section  31,  193. 
section  34,  79,  85,  96. 
section  2, 139-140. 
section  10, 130, 146. 
section  17,  144, 145,  146. 
section  20,  146. 
section  21,  144-145. 
section  27, 146. 
section  11,  139-140. 
section  25,  97. 
section  26,  97. 
section  34,  97. 
section  35,  72,  99. 


2. 


section  19, 
section  20,  72. 
section  30,  72. 
section  31,  189. 

section  2,  72. 
section  3,  97. 


314 


INDEX. 


Rauge  24  W.,  T-  5i5  N..  section  13,  90.  154. 1«9. 
section  15,  40, 12i. 
section  24,  189,  190. 

25  W\,  154.  295. 
26\V.,  154. 

26  W.,  T.  55  N..  section  22,  189. 

secftion  23,  189. 
Rattle  and  Nye,  analyses  by,  221. 
Recrystallization  of  chert.  159,  273. 

of  Lower  Huronian  graywacke.  75,  S3-84. 
Red  River,  lobe  of  ice  sheet.  194. 
Red  rock,  of  Keweeuawan,  57,  58. 
Renard,  referred  to.  239,  242.  253,  254.  255.  259. 
Reserve  tonnage  of  iron  ore  in  Mesabi  district,  290. 
Rhyolite,  porphyritic.  of  Archean,  63:  described,  68-69. 

of  Lower  Huronian,  described,  7&-S0. 
Rivers  of  Mesabi  range,  21-22. 

(See  Mississippi,  Prairie.  Swan,  Embarrass.  Pike.    Par- 
tridge. Dunka.) 
Rivot,  referred  to,  277. 
Roberts  mine,  analyses  of  iron  ores  from.  216 

Pokegama  conglomerate,  96. 

shipment  from.  287-288. 

transportation  of  iron  ores.  285. 

underground  mining  in,  282. 
Robertson,  R.  S.,  analysis  by,  223. 
Rogers,  analyses  by,  241. 
Rosenbusch,  referred  to,  172. 
Rouchleau  mine.  43. 
Rust  mine,  anjilysis  of  water  from,  264. 

depth  of,  208. 

shipment  from.  288. 

transportation  of  iron  ores,  285. 

underground  mining  in,  '282. 
Rutile  of  altered  Lower  Huronian  rocks,  S3. 
Saganaga  Lake,  granite  of,  intrusion  in  Lower  Keewatln,55. 
St.  Croix  Valley,  Hinckley  sandstone  of.  relations  to  Kewee- 
uawan, 57. 
St.  Johns  group  of  New  Brunswick,  equivalence  to  iron  ore, 

57. 
St.  Lawrence  River,  21,  22. 
St.  Louis  River.  21,  22,  25. 

graywacke  and  slate  on,  50.  54,  203,  204. 
Sand  in  iron  ores,  211,  296. 
Sand  Mountain,  Clinton  iron  ores  from,  photomicrographs 

of.  250. 
Sauntry- Alpena  mine,  analysis  of  iron  ores  from.  217. 

dip  at.  179,  225. 

shipment  from,  288. 

steam-shovel  mining  in,  280. 

structural  relations  of  iron  ore,  22b. 

surface  of  iron-ore  deposits,  226. 

transportation  of  iron  ores.  285. 

view  of,  294. 
Schists.    (.SeeHornblendic  schists.  Chloritic  schists.  Micace- 
ous schists.) 
Schoenfuld,  analysis  by.  242. 
Schoolcraft.  H.  R.,  referred  to,  25. 

summary  of  literature,  32. 
Slichter.  C.  S.,  referred  to.  267. 
Seaman,  A.  E.,  referred  to,  211,  275. 
Sebenius,  J.  U,,  indebtedness  to,  20. 
Section.    {Sec  Range. ) 
Sections,  subdivision  of.  291. 
Section  33,  mine,  shipment  from,  288. 

irunsportalion  of  iron  ores,  285. 
Security  miiR-.  43,  41. 
Sellers  mine,  depth  of.  20n. 

(Shipment  frv>m.  2.s7.  289. 


Sellers  mine,  transportation  of  iron  ores,  285. 

underground  mining  in,  282. 
Serpentine  of  Biwabik  formation,  137. 
Shale  of  Cretaceous  formation,  189. 
Sharon  mine,  limonite  in,  210. 

milling  in,  282. 

shipment  from,  289. 

transportation  of  iron  ores,  285. 

■\iew  of  stripping  operations.  286. 
Sharpless  and  Lane,  referred  to.  142. 
Shipments,  from  Marquette  district,  289. 

from  Menominee  district,  289. 

from  Mesabi  district,  29.  287-239.     * 

from  Vermilion  district,  '289. 
Shipping  points,  sketch  map  of.  19. 
Short  Line  Park,  rocks  of.  57.  58. 
Sidener,  C.  F.,  analysis  by,  139-140,  221.  222,  223. 
Siderite.    (See  Carbonate.) 
Sideritic  slate.  Birch  Lake,   described,  150-153   (specimen 

45161). 
Silica,  abstraction  from  iron  ore,  262,  264. 

in  iron-ore  deposits,  210,  219. 

powder,  analyses  of,  210. 
Silliman.  A.  P..  referred  to,  212. 
Sills,  diabase  in  Upper  Huronian,  58,  185,  186. 
Sioux  quartzite.  assignment  to  Potsdam,  57. 
Slate  of  Biwabik  formation,  101;  described  143-148;  plate  of 
152  (specimens  40741,  40742,  40863,  45003,  45006.  45009, 
45039.  45175,  45191,  45224,  45228,   45391,   45461,  4-5541, 
45591.  45592.  45593,  45594,  45600,  45625,   45630,    45639, 
45645,  45652,  45670,  45672,  45672A,  45677,  45678,  45699, 
45734,  45737,  45780). 

alteration  to  paint  rock,  149;  plate  of,  152  (specimen 
45587),  234. 

as  a  guide  to  exploration.  297-298. 

of  Birch  Lake,  analysis  of.  153  (specimen  45161). 

relation  and  comparison  with  Virginia  slate.  172-176, 
297-298. 

relations  to  iron  ore  deposits,  212,  219.  227,  229.  263.  266, 
267,  271,  272,  279. 

{See  Biwabik  formation.) 
Slate  of  Carlton  and  Cloquet,  203. 

of  Gunflint  Lake,  169,  177. 

of  Lower  Huronian.  described.  74.  75, 76  (specimen  45494). 
(Sec  Lower  Huronian.) 

of  Penokee-Gogebic  district.  177.  ISO. 

pebbles  in  Lower  Huronian  conglomerate,  76 
Slate.  Virginia,  described,  168-177. 

(.See  Virginia  slate. ) 
Slicing  system  of  mining.  282-283. 

{See  Mining  methods.) 
Smyth;  C.  H..  referred  to,  250,  251. 
Smyth,  H.  L..  referred  to,  118.  277. 
Soda,  in  glauconite,  243. 

in  greenalite,  243, 
Soil  of  Mesabi  district,  22. 
South  Dakota,  Keewatin  sediments  of,  63. 
Sparta  mine,  analyses  of  iron  ores  from,  217 

depth  of,  208. 

dip  of  iron  ore  deposit  layers,  225. 

limonite  in,  210, 

shipment  from.  287,  289, 

steam-shovel  mining  in,  280. 

structural  relation  of  iron  ore,  229. 

texture  of  iron  ore.  224. 

transportation  of  iron  ores,  285. 
Spai:ta  mine,  underground  mining  in,  282. 
Sparta  (town),  Archean  rocks  near,  63,  65,  76, 

railway  connections.  23. 


INDEX. 


315 


Sperry,  E.  A.,  inrlebtedness  to.  20. 

referred  to,  1.54. 
Sphene,  of  Embarrass  granite,  187. 
Spherulites  of  epidote,  118. 
Spherulitic  texture  in  basalt.  65. 
Spilosite  from  Crystal  Falls  district,  174. 
Spruce  mine,  analyses  of  iron  ores  from,  215,  217. 
depth  of,  208. 

slate  in  Biwabik  formation,  analyses  of,  144-145  (speci- 
men 45678). 
shipment  from,  289. 
texture  of  iron  ore  in,  224. 
transportation  of  iron  ores,  285. 
underground  mining  in.  282. 
Spun,  J.  E.,  referred  to,  14, 17,  30,  51,  52,  60,  114-115,  116,  120, 
139-140,  144-145,  1-59,  210,  239,  241,  247.  278,  279- 
summary  of  literature,  45,  60. 
Stanton,  T.  W.,  on  Cretaceous  fossils,  190. 
Staurolite  of  altered  Lower  Huronian  rocks,  83. 
Steam  shovel  mining,  280-282,  294. 

{See  Mining  methods.) 
Steiger,  Geo.,  analyses  by,  139-140,  141,  144-145.  150,  153,  245. 
Stephens  mine,  analyses  of  iron  ores  from,  217, 

steam  shovel  mining  in,  280. 
Stevenson  mine,  analyses  of  iron  ores  from,  217. 
depth  of,  208. 
dip  at,  178. 

dip  of  iron  ore  deposit  layers,  225, 
shipment  from,  289. 
steam  shovel  mining  in,  280, 
transportation  of  iron  tres,  285. 
Stevenson  (town),  railway  connections, 23. 
Stokes,  H.  N.,  analyses  by,  139, 140,  144-145, 150,  171-172. 
Stone  mine,  exploration  near,  40. 
Stony  Brook  Junction,  railway  connections,  28. 
Strike  of  Lower  Huronian,  Sd. 
Succession  in  Mesabi  district,  36,  51. 
Sulphur  in  iron  ore  deposits,  21S,  219. 

(.See  Iron  pyrites.) 
Superior,  Wis.,  docks  at,  285. 
Swan  Eiver,  21,  26. 

railway  connections,  29. 
Syndicate  Company,  Mesabi,  44. 
Taconic  series,  36,  38,  39,  40,  43,  .52,  55-56. 
carbonate  In,  40. 
iron  ores  of,  40. 
relations  to  Archean,  52. 
use  of  term,  27,  202. 
Taconite,  42,  47,  248.  i 

use  of  term,  42,  101. 
volcanic  nature  of,  59. 
('See  Ferruginous  chert.) 
Tertiary  sediments,  absence  of  in  Mesabi  district,  199. 
Test  pitting,  300. 

Thunder  Bay,  Animikie  of,  27,  201. 
Thomas  Lake,  Biwabik  formation  near,  52. 
Till,  of  Pleistocene,  192. 

Titaniferous  magnetic  iron  of  Duluth  gabbro,  183,  223. 
Todd,  J.  E.,  referred  to,  194. 
Tonnage,  reserve  of  Mesabi  iron  ores,  290. 
Topography  of  Mesabi  district,  20-21;  summary  of,  13. 
Tourmaline  of  altered  Lower  Huronian  rocks,  S3. 

of  Embarrass  granite,  187. 
Tower,  Lower  Keewatin  greenstone  near,  53. 
Townships.    (.See  Ranges.) 
Towns  in  Mesabi  district,  23. 

(See  Biwabik,  Buhl,  Chisholm,  Eveleth,  Grand  Rapids, 
Hibbing,  McKinley,  Mesaba,  Mountain  Iron,  Nash- 
wauk,  Sparta,  Virginia.) 


Transportation  lines,  sketch  of,  19. 

Transportation  of  Mesabi  iron  ores  described,  43,  285,  286. 
{.See  Duluth  and  Iron  Range  Railway,  Duluth,  Missabe 
and  Northern  Railway,  Eastern  Railway  of  Minne- 
sota.) 
Tripoli  powder  in  iron  ore  deposits,  210. 
Tuffaceous  texture  in  basalt,  65. 
Turgite  of  iron  ore  deposits,  209. 
Turner,  referred  to,  66. 
Two  Harbors,  docks  at,  285. 
Unconforniity  between  Animikie  and  Keewatin,  55. 

between  Archean  anC  overlj'ing  rocks,  23,  52,  70-71,  86, 

195. 
between  Cretaeeous  and  Keweenawan,  23. 
between  Keweenawan  and  underlying  series,  197,  198. 
between  Pleistocene  and  Cretaceous,  23. 
between  Upper  and  Lower  Keewatin,  52. 
between  Upper  Huronian  and  the  underlying  series,  23, 
86,  94-98, 180-181,  196. 
Underground  mining.    (See  Mining  methods. ) 
Union  mine,  analyses  of  iron  ores  from,  217. 
shipments  from,  289. 
transportation  of  iron  ores,  285. 
underground  mining  in,  282. 
United  States  Board  on  Geographic  Names  referred  to,  21. 
United  States  Steel  Corporation,  indebtedness  to,  20. 

iron  ores  controlled  by,  285,  286. 
United  States,  iron  ore  production  in,  289. 
Upham,  W.  N.,  referred  to,  50,  192,  194. 

summary  of  literature,  49. 
Upper  Cambrian  of  St.  Croix  Valley,  relations  to  Keweena- 
wan, 57. 
Upper  Felch  Mountain  series,  equivalence  to  Upper  Huron- 
ian, .50. 
Upper  Huronian,  201;  described,  88-205. 

absence  of,  north  of  Mesabi  range.  196-197. 

correlation  of,  30,  36,  41,  50,  201,  202-205. 

deposition  of,  196. 

distribution  of,  24,  66,  70,  73,  80,  88,  89,  91,  184,  203. 

folding  of,  197,  198,  204,  260. 

initial  dip  of,  197. 

of  the  Lake  Superior  region,  200. 

relations  to  other  series,  23,  35,  42,  70-71, 180-181, 187,  188, 

196,  197-198,  260,  272. 
shore  line  of,  196. 
structure  of,  15, 178-180. 
thickness  of,  15,  55,  180. 
sketch  of,  near  Mailman  camps,  80. 
sketch  of,  near  Hibbing,  66,  70. 
summary  of,  14-15. 
Upper  Keewatiu.     (.See  Keewatin. ) 
Upper  Marquette,  correlation  of,  41,  50. 
Upper  Vermilion,  correlation  of,  41,  50. 
Utica  mine,  shipment  from,  289. 

tr-^nsportation  of  iron  ores,  235. 
Van  Hise,  C.  R.,  indebtedness  to,  19. 
letter  of  transmittal,  11. 
referred  to,  30,  31,  34,  44,  45,  115,  118,  128,  134, 142, 159, 174, 

200,  201,  248,  255,  256,  257,  258,  263,  264,  277,  279. 
summary  of  literature,  41,  50. 
Van  Hise,  C.  R.,  and  Leith,  C.  K.,  summary  of  literature.  60. 
Vein  quartz  in  iron-ore  deposits,  212. 
in  Lower  Huronian,  83. 
in  Pokegama  conglomerate,  95,  97,*  98. 
in  Upper  Hiu'onian  series,  179-180. 
near  contact  with  graiiite,  83. 
Vermilion  district,  Archean  sediments  in,  77. 

correlation  of  iron-bearing  formation,  36,  39,  40,  64. 
ellipsoidal  greenstone  in,  53,  65. 


316 


INDEX. 


Vermilion  district,  erosion  of.  264. 

iron  ores  of.  36-37. 130.  277.  293-294.  299. 

Monogrrnph  on,  11.  61. 

porphyries  of.  68-69. 

shipment  from,  289. 
Vermilion  series,  origin  of.  45. 
Victoria  mine,  shipment  from,  289. 

Iransporiation  of  iron  ores,  285. 

underground  mining  in,  282. 
Virginia  itown).  178. 

analyses  of  chert  and  slate  from.  139-141.  170-171. 

discovery  of  iron  ore  near,  28. 

drainage  near,  234,  235. 

faulting  near,  46,  52. 

railway  connections,  23.  28. 

rocks  near.  63,  6S;  photomicrograph  of,  174;  plates  of,  72, 
104.  139-140.  141,  171. 

texture  of  iron  ore  at,  223. 
Virginia  mine.  43,  44. 

Virginia  slate,  described.  16S-170:   summary  of.  15;   photo- 
micrographs of,  174  (Specimen  45463). 

analyses  of.  170  (specimens  45463,  45735,  45767). 

cordierite  hornstones  in.  described,  171-172. 

deposition  of.  196. 

diabase  sills  in,  56. 

erosion  of.  197,  199,  260.  . 

influence  on  circulation  of  water  and  concentration  of 
iron  ore.  177,  179,  266,  267,  271,  279,  297. 

iron-ore  deposits  under,  206-207,  297. 

limestone  of,  56;  described,  171. 

nodules  in,  137. 

relations  and  comparison  toBiwabik  formation,  47,  172- 
176,  196,  297-298. 

relations  to  other  formations,  42.  55,  182. 

thickness  of,  46,  56,  177.  180,  260. 
Volcanic  sand  altering  to  iron  ore,  56. 
W'acoutah    property,  paint    rock    of    Biwabik  "formation 

analysis  of,  149-150. 
Walcott,  C.  D..  referred  to,  257. 
Warren,  O.  B.,  referred  to.  62. 
Water  in  iron  ores.  218,  219,  235. 

in  paint  rock,  223. 


Weathering  of  gabbro.  182. 

Wedding,  H.,  referred  to.  62. 

Weed,  L.  B.,  indebtedness  to,  20. 

West  Duluth,  docks  at.  285. 

Western  Jlenominee  series,  equivalence  to  Upper  Huro- 

nian,  .50. 
Western  New  England  iron  ores,  assignment  of  Taconic 

to,  39. 
Wilkinson,  C.  D.,  referred  to.  62. 
Williams  mine,  shipment  from.  287.  289. 
Willis.  Bailey,  summary  of  literature.  37. 
Wills  mine,  shipment  from,  289. 
transportation  of  iron  ores,  285. 
underground  mining  in.  282. 
Winchell,  H.  V.,  referred  to,  29,  30,  62. 159,  190,  191.  218.  278. 
summary  of  literature,  37,  38,  39,  41. 
1  Winchell,  N.  H.,  referred  to,  26,  27,  29.  30.  34,  36.  55,  62,  159. 
171,  183,  221.  222,  223.  278. 
summary  uf  literature.  34.  35.  36,  37.  38,  39,  43,  44,  52,  60. 
Winchell,  N.  H.,  and  Grant,  f.  S..  summary  of  literature.  50. 
Wisconsin  epoch  of  Pleistocene,  191. 
Wisconsin  Valley  series,  equivalence  to  Upper   Huronian. 

50. 
Wisconsin  zinc  ores,  comparison  with  development  of  green- 

alite  and  carbonate.  258-259. 
Wisconsin.    (See  Commonwealth,  Black  Kiver  Falls,  Bara- 

boo.) 
White,  C.  A.,  on  Cretaceous  fossils,  190-191. 
White  Iron  Lake,  granite,  similarity   '^  Lower  Huronian 
granite,  188. 
Lower  Keewatin  greenstone  of,  53. 
Whittlesey.  Chas.,  referred  to,  25. 

summary  of  literature.  33. 
Woodbridge,  D.  E.,  referred  to,  62. 
Wright,  referred  to.  277. 
Wyoming  mine,  43. 

Xanthosiderite  of  iron-ore  deposits,  209. 
Yellow  ocher.     {See  Limonite.^ 
Zinc  ores  of  Wisconsin,  comparison   with  development  of 

greenalite  and  carbonate,  258-259. 
Zirkel,  referred  to,  172. 
Zoisite  of  Biwabik  formation,  160, 163. 


O 


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[Monograph  XLIII.] 

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4°.     316  pp..  33  pis. 


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United    States   geological   survey  |  Charles    D.    Walcott,    di- 
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4°.    316  pp.,  33  pis. 

[United  States.    Department   of  tlie    interior.     ( L'.   S.    ijeotogical   survey.) 
Monograph  XLIII.) 


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[United   St.\tes.    Department  oj  ilie   interior.     {V.     .S.    geological  survey.) 
Monograph  XLIII.] 


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